We use Solupay, and we've used it no problem with "cash sales" that involve a Credit Card. We've also used "E-Invoices" that are an off-shoot of invoices. I'm not sure how well known it is, as I think it's a fairly new feature. However, we use it every day, and we've got it working pretty smoothly. Once the form layout is set up, and when we have an order that's saved, an email (Advanced PDF) is sent to the customer who pays it, and then we have some saved search triggers set up to let us know when they're paid, so that the order can proceed in processing. Then all we have to do is hit the "bill" button, and save when the form loads. All is done mostly "automagically", but we had to have help from Solupay's dev people to make it work. However it works nicely.