varktech.com on "[Plugin: WooCommerce - excelling eCommerce] Shopping Cart Empty at Checkout"

Hi,
Sounds like an environment issue, possibly a plugin/theme conflict. If you don't have one already, suggest creating a testing environment. In that environment, suggest spending the time until you can reproduce the issue at will. Then you can track whether it's a module conflict by deactivating all but woo, switching to a clean default theme like 2014, and testing. If good, add back in 1 module. Rinse and repeat until conflicting module identified.
Best,
Vark