My advice, is to not build that in just yet. Say you charge, tell them the price, and take their credit card or money - and put them on a plan. Manually charge them through Square or whatever the first month or two, then build that portion out as it's required. 37Signals and a whole other slew of companies did just this too - so I'm not crazy.

The real reason to delay it personally, is that you might find their acceptable industry payment is different than you think. They may be used to paying monthly via check, and not do CC's. You never know. But this gives you the ability to charge, AND figure out the correct long term solution.