The problem has been solved (that all my invoices were lacking the payment button). Was apparently a system-wide software bug:
Thank you for contacting CartManager.
The problem that was occurring was due to you only having Google Checkout as the only payment method. However, our developers were able to recognize the issue the cart was having and it has been resolved.
If you resend the invoices to your customers they should be able to checkout without any problems.
I asked cartmanager if they would extend my 14 day free trial, starting from today, as a concession for all the headaches. | 
