Paypal not checking out for multiple items in cart

I'm having an issue with my site at https://www.celestialtoystore.com

Customers ordering multiple items or more than one of the same item can't seem to check out with Paypal. When they check out they get this screen:

However single items check out fine. Does anyone know if this is a Paypal problem or I'm doing something wrong in my store configuration?

Any ideas would be much appreciated.

It is really difficult to say. The best solution would be to upgrade the cart to one that is significantly newer, more secure, and known to be able to handle such simple matters.

A recent post about a ZC 1.3.8a site contains several good reasons why one should use store software that is younger than 10 years old and all of the security concerns about using that particular version.

You may receive specific help on how to keep this functional, but really should be looking at doing an upgrade by rebuild where ultimately if/when done correctly you can still have the look and at least important operation but built on far more secure and modern software.
http://www.zen-cart.com/entry.php?3-How-do-I-rebuild-my-site-on-the-new-version-instead-of-upgrading

I forgot to add that I am using the Paypal IPN payment module. Paypal Express Checkout appears to work fine.

PayPal has continuously iterated the requirements over the past 10 years since ZC 1.3.8 was released. If you want your site to keep working, you need to keep upgrading too.

If you won't upgrade, then you may need to skip using the IPN module, and switch to Express Checkout. But even that implementation in v1.3.8 is obsolete now too.

In short: time to upgrade.