PayPal on Wednesday made its big move to upend Google (GOOG) Wallet in the mobile payment platform wars by signing a deal to enable its mobile payment system on Discover’s credit card network. As reported by GigaOM, the new partnership will deliver PayPal’s in-store payment service to more than 7 million different stores across the United States starting next April. GigaOM says that when the service launches next year, PayPal customers will be able to use a PayPal Access Card to buy in-store goods in conjunction with PayPal’s mobile wallet app, which delivers deals and handles receipts, among other things.