W3 Total Cache is a great caching plugin, but sometimes it can interfere with payments processing. For this reason we recommend you add an exception for Sliced Invoices in your cache settings.
To do this, go to the Performance -> Page Cache settings page. In the “Never cache the following pages” box, add the following:
(Note: this assumes your invoices URL slug is the default “sliced_invoice”. If you are using the Better URLs extension to change it, enter your new slug here instead.)
The end result should look like this: