When you sign up for ProPay they give you a default amount that you can charge per month and per transaction. Normally, this is not enough for all your tenants. You will need to request a limits increase by emailing firstname.lastname@example.org to request getting higher limits.
Since they "front" money so that you get your money quicker they will need more credit information about your business in order to get higher limits. This typically means incorporation papers, Federal Tax ID, etc. It's really like adding a line of credit so they will want this extra information.