Wishlist member and 1shoppingcart integration returning page not found

Hello, I’m trying to get Wishlist Member’s 1shoppingcart integration to work and keep getting failed results:

I’ve setup the product in 1shoppingcart using the sku provided by WL, and and inserted the Thank You URL specified by WL into the 1shoppingcart product link, as well as the general thank you link.

When I go through the purchase process, the checkout piece from 1shopping cart works (I receive the confirm email from the purchase), but when it redirects back to my WP page I receive the generic PAGE NOT FOUND error rather than the next step of the process, which is to prompt the user to complete the registration.

I have:
– double-checked that the Wishlist Member generated page is there,
– ensured that all the links/sku’s are correct on both sides
– ensured that ‘Prevent duplicate shopping cart registrations’ is turned OFF (saw this in another support thread)

Possibly related info,
1) using WP 3.0
2) the WP site permalink settings is set to /%postname%/, and the Thank You URL provided by WL is structured like https://mysite/index.php/register/abc123.php (don’t see how it would matter, but it might…)
3) the 1shoppingcart service is the $34 shopping-cart only package, we were contact a few days after signing up telling us that our implementation needed the $94 service because it needed 1SC’s API integrations, WL tells us it does not (*Could anyone using WL/1SC let me know what 1SC plan you’re on?)

Any help on this would be very much appreciated. I’ve posted the support questions to Wishlist Member, but they’ve only given me pointers on how to double-check general settings, and their response rate has been kept to a slow 1 answer per day.

Regards,