'Opt-In' donation to cover CC fees?

Hello!

We are starting the process to add an opt-in at checkout so our patrons can add on a small donation to cover cc processing fees. Are there any Tessi orgs currently doing this? How has the response been? Are you on TNEW?

We are in the middle of upgrading into V.7, and were thinking of purchasing the Round up donation component that Tessitura offers, and retooling it for this purpose. 

I'd love any ideas or input that the hive mind can offer, thank you so much!