Generally, a dispute is when you can't contact the seller, or their proposed solution is not acceptable. There is no specific situation that would automatically warrant a dispute. Disputes rarely happen with good customer service, unless we're talking about overseas untraceable packages like I stated above.
You can dispute up until the 45 day window, after which, you can escalate to a claim at any time. However, after you make a claim, you have 20 days to work it out with the seller or it will automatically close. So you either let it close when you reach an agreement with the seller, or you escalate it to a claim. If either you or the seller opts to escalate to a claim. Paypal will take all information they have into account and decide whether reimbursement is required. Usually this is the step where you would want to provide tracking information and any correspondence.
According to Paypal you can open a dispute IF:
"You don't receive the item.
You receive an item but it's significantly different than the description on eBay or the seller's website."
if you want the FT to take care your issue by returning the DOA item and to protect yourself, just use registered letter and ask FT to reimburse the untracked mail cost. with some addition, you can feel peace of mind about the shipment.
I also agree with CalvinIS in some perspectives.
this is quote a explanation from Ebay CS regarding opening a case when I have passed 45 days window.