If they require proof of purchase, then there is a good chance that Chewy can provide your order manifest. I've been able to see my orders from 800PetMeds and Chewy for the entire time I've been a member (granted, only a year, but all of the orders are still there).
Any vet's office that treated your animals will also have records for purchases in your files most likely. You can probably call them and ask for a printout of the purchase orders.
But I've been in many class action lawsuits at this point and never had to prove my purchases even from years back. They may cap the amounts you can claim unless you have receipts for more, but you should be awarded some sort of settlement.