Angelica,
My name's Brandon and I work at Paywhirl. We're actually working on adding the capability of charging a customer at signup and then setting a date on which all your customers will be billed. If you're interested in using it and giving us some feedback on how it works for you, email me at brandon [at] paywhirl.com and I'll be happy to get your account setup. In addition, we're in the early stages of some deeper Shopify integration and we'd love to have you involved with that as well. :)
- Brandon Swift