That helps a bunch. It looks like you are missing a value in one of your add to carts that is on the page. Near the line on your choose_membership page referenced in the error you have this line of code:
$row_rsMemb1 = WAEC_findRecordMySQL($rsMemb1, "", $ATC_itemID);
You should search for this and update it to be like this:
$row_rsMemb1 = WAEC_findRecordMySQL($rsMemb1, "id", $ATC_itemID);
The missing 'id' text is what seems to be causing these errors. If you update this line of code to include the id then you should not get this error. Please give this a try and test it again to see if you get any more errors. Please let us know what you discover.