There's nothing you have to send in with form 8910. But as with almost all of your deductions and credits on your tax forms, the onus is on you to prove that you actually earned the credit or deduction if you ever get audited.

I'd just throw a backup copy of the purchase agreement in your file with your other tax paperwork so you don't have to go digging for it somewhere else in case you do ever get audited, but no need to do anything more than that.