The add to cart and quick cart are working for me using that link. Is it possible you are going from the www. version to the version without or vice versa? That would cause the cart to clear. I usually suggest using the .htaccess file to force users to use one or the other.
I don't suggest using the sandbox. You need to use a sandbox user and sandbox credentials and it can get confusing. Just use a live account and refund the transaction. Paypal will refund all fees and you can do as many refunds as you want.