EMERGENCY! something not right with payment module - Simple SEO URL?

[Xbox Memberships]

So everything has been working fine after deleting the cache for the simple seo url module a customer phoned up. they had gone to make a payment and they got a 404 error? now a friend has generasly tested this and it all went through fine. i removed google checkout before the test.
so i rang the customer up and asked him to try again. he did and it gave him a 404. so i then logged in and tried to use a spoof set of card details as i thought it may be a failed card and wanted to recreate the error but it just redirects me to the main page? there is no record of an attempt to pay either as the system records failed transactions.

this error is a little frustrating as the customer may think there is an error with my site rather than the card so they may just leave the site instead of trying a different card

[CoolCarPartsOnline]

disable the SEO on the checkout page. Also it could be due to some server changes or something you changed on the website.

[Xbox Memberships]

how do i disable the SEO module on the checkout page?