Determine standardized volume pricing for ACH credits

[jkwade]

UPDATE: changed title to address volume pricing for debits as well

Per the discussion in #48, I am creating a new issue to determine if there should be volume pricing tiers for ACH credits, and if so, what those tiers will be.

Balanced currently charges 25¢ per ACH credit (i.e. each time we pay a merchant via next-business day ACH)

The pricing would work in the following way (per month). This is the standard model for tiered pricing:

1. up to X credits : 25¢
2. Y > # of credits > X: YY¢

3. of credits > Y: ZZ¢

There are two steps required for this task:

1. determine the tiers (no more than what is listed above)
2. determine the pricing for each tier

[matin]

The feedback has been that 25¢/transaction is an acceptable rate. Closing this issue until there's enough feedback on requirements for discounted ACH credit pricing.