Sum values in related modules

Hello -

I've seen several threads/discussions about retrieving field values from related modules but not this - a calculation/summation of a field in a related module.

We created a custom module that tracks things customers purchased from us. One of these things is software licenses. A customer might buy 5 licenses today. Last year, they bought 10 and the year before, they bought 15.  That means they have a total of 30 licenses. The list of "things" they bought from us could be substantial.

We want to put a calculated field on the Account Record View Layout that shows the total number of licenses this account owns. We don't want our staff to have to manually page through all the things and add up the number of licenses.

So I'm wondering if there is a formula that can access "license" records in the "Purchases" module where licenses are not zero, sum them, and return them to a field in the Accounts module? 

I haven't seen this specific request although I've seen some similar threads but not been able to find anything that works.

Thank you very much!