The only way to have your client's tangible products appear in the same cart as the downloadable ones which will be sold through our service would be to use our cart for everything. Our features depend on the use of our cart, so there's no way to integrate with a third party cart and handle only a portion of the order.
How many products will your client have in total, downloadable and tangible? Often it does not much more, if anything at all, to use our cart for everything. Our prices are based on the number of products being sold and the amount of digital storage needed.
For example, if the client was selling five 10 MB files they could use our $5 plan, which supports 10 products and 50 MB of storage space. They'd have filled up their storage space with those five digital products, but still have room for five more non-digital products without requiring an upgrade.