I haven't been able to find help for this anywhere else, but after I put in my payment info, (I'm buying three of the mutator promotion games along with all three mutators) it says, "order limit exceeded". I've tried several times to no avail. Any help would be appreciated. Thanks

Try reducing possible errors on your side by logging out of GOG and deleting your browser's cache. You can even reboot your system as well as cut and reestablish your Internet connection (e.g. by rebooting your router/modem).

If the issue persists, use this form to contact support:
http://www.gog.com/support/contact/my_orders_and_payments

EDIT: GOG has got a rule in place, that allows to buy the same game only five times per day. This limit is to prevent resellers to buy dozens of keys. Every normal customer having issues with this is encouraged to contact GOG support.