AppleCare+ must be purchased within 60-days of initial purchase/device activation. Eligibility to purchase can be established from iPad Settings. You should see the AppleCare option visible in iPad settings - this usually appearing just below your Name / AppleID.
As for acceptable proof of entitlement, Apple typically expect an original sales invoice that shows the serial number of the device. Whether or not an aftermarket retailer's invoice will be accepted is question that can only be answered by Apple.
Contributors here are all end-users, just like you - and cannot provide an authoritative answer. You would be best advised to direct your question directly to Apple Support. You can contact the Support Team using the Support link at top-right or bottom-left of this page.
Alternatively, you can initiate and manage your support cases from Apple devices using the excellent Apple Support App. If not already installed, the App can be downloaded from the App Store:
https://apps.apple.com/app/apple-support/id1130498044