I did not actually have to file a claim, since they found my package eventually, but here's what their website says:
3.2 Proof of Value
Either the mailer or the addressee must submit acceptable proof to establish the cost or value of the article at the time it was mailed. Proof of value should be submitted electronically or attached to the claim form under 1.5; otherwise, the claim cannot be processed. Other proof may be requested to help determine an accurate value. Examples are:
A sales receipt, paid invoice or bill of sale, or statement of value from a reputable dealer.
Paid repair bills; if the claim is for partial damage, estimates of repair costs or appraisals from a reputable dealer. Repair costs may not exceed the original purchase price.
Receipt or invoice for the costs incurred to buy a surety bond required to reissue a lost item.
Receipt or invoice of costs incurred for the reconstruction of nonnegotiable documents.
A copy of a credit card statement or other documentation indicating the amount paid.
For Internet transactions conducted through a Web-based payment network that offers payment services through a stored value account, provide a computer printout of the online transaction identifying the purchaser and seller, price paid, date of transaction, description of item purchased, and assurance that the transaction status is completed. The printout must clearly identify the Web-based payment network provider through which the Internet transaction was conducted.
None of which would be applicable to comic books bought long ago.