Where this extra 5% most likely came from is
Leviticus 27:31 And if a man will at all redeem ought of his tithes , he shall add thereto the fifth part thereof.
Of course, this has nothing to do with "an offering" above the required tithes. However, I recommend reading from
Leviticus 27:24-33. Now, in order to get a better understanding of this passage and how it should be implemented in one's life, I would suggest seeking out the teaching of an established rabbi. That is, of course, for those ministers who really believe in following the law of tithing, and are willing to implement and live by all ten of the tithing ordinances.
If not, then go ahead and demonstrate to the assembly how to pick and choose which laws, ordinances, and statutes one wishes to follow and which ones you will not follow. Even so, please pay careful attention to the rules of selection that you have created, so that the entire congregation can also follow your example concerning law selection. Also, one will have to be very clear in these kinds of matters in order to justify one's self before God when He asks, "What have you done with my laws, that you should sit in judgment of them?"
Most people fail to look past the perceived immediate benefit they see in imposing some laws and dismissing others. Few, if any, actually realize that when one manipulates the word/laws of God, they are attempting to recreate God after their own image and in their own likeness. They may even love this god of theirs, but they certainly don't fear him.