How can I give purchasers the option to make monthly payments?

How can I give purchasers the option to make monthly payments?

I assume you're aiming to setup recurring payments. Most payment processors can be used for this and we have a general guide on how to set up one.

Complete guide: How-to-Setup-a-Recurring-Payment-Option

Have a quick look at it, give it a spin, and let us know if you need further help along the way.