If I were in your shoes (and I've been at this exact stage), I would manually bill until I had enough clients to worry about it. You can do a half and half app

If I were in your shoes (and I've been at this exact stage), I would manually bill until I had enough clients to worry about it. You can do a half and half approach. Let the user know that they will be invoiced, and a representative will reach out to take their payment information. For example, if I was using Stripe (the payment processor I'm familiar with), I would setup my register button to just send me an email with data that I could then enter into Stripe manually, ie. name, email address, etc. I wouldn't include CC info in this because I wouldn't want to deal with compliance. Since you told them to expect a call and you're invoicing them you've got a good reason to call. If you go this route let the user use your product right away. The other cool thing about this approach is that you can also start to build a rapport with your clients, which pays dividends when you want to solicit feedback. I find that motivation really spikes when you start seeing sales and money rolling in.