Here are a few things to try:
If the error is occurring after you've entered your payment info, then its likely a typo in your billing address that you entered on our site (particularly your house number and/or zip code, check for spacing errors). If those don't match exactly what you bank has on file for you, the payment will not go through.
If you're checking out via Paypal and are receiving a gray screen, please close your browser, clear you cache and go directly to our site, www.allinonepaint.com, in a new browser. This error typically occurs when you've come to our site via a Facebook link. Paypal and Facebook are not playing well with each other.