I suspect the head of WT90 weighs More than 13 OZ.First Class limit. If so, using your own envelope and trying to pay $5 or $6 does not seem possible .
Unless its shipped 3rd or 4th class…the only way to go is priority mail The Large Flat rate padded Envelope the most economical if it can fit in there.
Finally finished the WT90 prototype mods, and got the best #'s I've seen so far from either WT90. Summary of the mods:
Plan was to double up 18 AWG LED wires but couldn't get them to fit, so backed off to just one pair.
ON 40T flat tops charged to 4.21V:
Even though it dropped nearly 1000 lumens in 30 secs, it seems more stable then previous measurements, holding high output better. Thermally it seems to be responding better, recovering quicker.
Great work Tom.
I am going to duplicate that success story.
Problem is, I do Not know how and my gifts and skills are elsewhere! 
Of course it depends on metal composition but I have gotten lucky with most of them, and they usually stay stretched unless further abused. I agree with all of your points, just chiming in.
I’ve decided to keep mine in the tube ‘USB side-up’ for easier access to the charging port, i don’t need to take the carrier out to get to it this way. If you are able to do this the spring is under less pressure so may be less likely to compress to the point where neither side will work. It may make no difference though if it is deforming due to heat rather than just settling down.
That’s a generous offer 
Given you have more intimate knowledge of the carrier than i do, is it easy to dismantle and do you see a way to extend the centre column at the USB end?
Just had it mind that it would be easier all round if Cochise could mail just the centre column for you to solder a button onto (for example) so that the contact point is at the same height on both sides of the carrier. I don’t know enough to know at which point this idea falls down though.
For example, if the spring is going to deform no matter what then it is the spring that needs attention, adjusting the centre colunmn will make no difference.
The recessed center column nut is not the problem - it's the spring, so best way to fix it is adding a bypass. If the spring is too deformed, I'd look at replacing it as well, but not sure if I have anything quality in that size and same stiffness - might, but not sure. If a got a "Blue" spring that fits, definitely would use it -- we know these springs well (https://budgetlightforum.com/t/-/51885, https://budgetlightforum.com/t/-/54402, https://budgetlightforum.com/t/-/56987). I think I have a supply of every size he has sold.
It's interesting though the WT90 stock driver springs looks very similar to the silver coated BeCu springs Blue sells.
For reference, djozz's spring tests: https://budgetlightforum.com/t/-/29825
Tom E. Modded carrier on LEFT.He swapped for mine because he is a nice guy and wanted to help eliminate the parasitic drain I was having. 
and I am sure he will make the swapped one better.
Besides bypass he disabled the USB to help eliminate parasitic drain,Used 20AWG wires to improve inside circuitry ,Lubed posts and screws with NO OX ID and sanded battery + brass button contacts for better contact.
Thanks Tom…mine( yours now) is being shipped after lunch. 
So it seems to me the next logical test is to compare the same modded proto head and test back to back with the stock carrier and modded carrier. The FET might help a smidge, partially offset by the wire downgrade. Thermal improvements likely have minimal turn-on impact, but since your measurement isn’t instantaneous, it’s a toss up to me which mod is responsible for the majority of improvement. Thermal or carrier mods. UCLp in other tests showed to be negligibly different than the production AR lens IIRC.
Off topic
In case anyone is wondering about my Avatar.
My oldest Brother took that photo on April 12,1969 at the Spectrum in Philadelphia .
Jimi Hendrix
JaredM - Yep, I was think'n the same thing, to isolate the changes to see. Problems I've had testing this light is that I have to give long cool down times, otherwise it reads much lower. It can take hours. I don't think happens any more on this one.
For example, I tested throw on the same cells I used with the lumens 30 sec test. I waited like 3-4 minutes, maybe, for the throw test, ran it for 5 secs on turbo first before recording the max on the meter. So it's sort of approximating fully charged cells at 30 secs, well maybe... Still, the throw # was the best I recorded and if I did this test the same way before, the reading would be significantly lower. Thinking it's the better MCPCB contact with the shelf.
But also the lumens at start was the most I recorded, and that is probably more attributed to the carrier and FET.
Ohh - the stock lights must be 20 AWG wires, not 18. 2 18 AWG wires will no way fit in the drilled holes and MCPCB cutouts. The stock wires are definitely thinner than any 18 AWG, though insulation thickness can vary, I don't think it explains it.
Time is my problem - got a lot of things goin on right now.
Very good results Tom! Thanks for all the testing.
If you need to get measurements back to back, I found that sticking an ice cube on the fins for a bit will cool the whole light right back down nice and quick.
I’ve experienced the same ‘cold-start’ behavior as well, on much smaller lights in my case. Even when it seems fully cooled back to room temp after say 10min, the turn on numbers are always suppressed on subsequent runs within say 30 minutes of each other.
Like TA said, I run the head under cold tap water to ‘reset’ things when doing reflector tuning for example. I do my best to keep water off the lens, but 15-20 seconds is all it usually needs to be back to baseline.
I understand the time limitation. My eyes are waayyyy bigger than my stomach for this flashlight stuff. I often say I’m going to do things that never happen when it comes to the hobby. At the end of the day, my EDC and maybe a compact thrower or gun light is what I put 80% of my time into. And of that, most of it is chasing the ‘perfect’ beam and tint for each of these applications.
WT90
Woww, very nice beam shots.
How soon can we get a Silver WT90? Texas_Ace, any new news about the latest production of the second lot?
I do note that the Black WT90 is still available, and the price has come down below US$160. But I am resisting the temptation to grab it. I still want a Silver, instead of a Black WT90.
Thanks.I will have better ones in the Fall and Winter when the atmosphere has less debris. :sunglasses:
I got an update a few days ago that they think they were able to source the missing materials for the silver lights (won’t know until they actually arrive).
If they have the parts then it should only take a week or so to finish making them.
Screen shots ZOOMED in.
End point/ Target ~ 120 yards.
Thanks Texas_Ace for the encouraging news.
BTW: Will production of the second lot take into account the technical glitches and their solutions that you and a few other experts have been discussing over the past few days?
As a greenhorn flashcoholic, I will not be able to do whatever necessary technical adjustments in order to enjoy the WT90 to its fullest potential.
The only outstanding issue I know of is the carrier parasitic drain and this can’t be fixed in this production run from the factory.
There's the very low glowing LED, apparently from the FET, though probably not as significant as the carrier issue.