If the vendor allows you to charge fees for overdrafts on savings accounts, I would categorize the related Regulation E related programming as being required by "compliance;" i.e. you are not asking for an enhancement that you should be asked to pay for.
As Doug demonstrated, Regulation E's application does not turn on whether the account is a transaction or savings account.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.