If the confirmation email you received post purchase includes the details below, it most definitely qualifies as a valid form of proof of purchase:
Date of purchase
Store information (store name, address, phone and/or email address)
Customer information (name, address, phone and/or email address)
Product which you have purchased coverage for (SKU and/or product description)
Repair or replacement plan which you have purchased (SKU and/or product description)
Sales prices for each of the line items
Feel free to forward that email message to cs@cpscentral.com.
You also can convert the email to a PDF file and email to cs@cpscentral.com, instructions can be found here.