Hi Mike,
Thanks for posting this on the community! This is not currently possible in Breww; however, there is a feature request open for One frees - cumulative retros (e.g. get one free for every 10 purchased) which I think to cover what you are looking for. Please check out that request, add your vote to help move this up the list and add any comments on how you would like this to work, which our dev team will review when they come to building this feature.
Let us know if you have any questions!