I am using Web Payments Standard (HTML button and code) and have sandbox accounts set up. Everything seems to work up to the point of logging in to a test PayPal account, or making a payment as a guest user, but then neither works - logging on as a buyer gives the error "Some of your information isn't correct. Please try again." and making a credit card payment as a guest user just says the payment has been rejected. Am I doing something wrong or does Web Payments Standard not work with a sandbox? Would I be better using PayPal Checkout?