Add Letter to Invoice Numbers
This can be done using the Invoice Number Override add-on available on The NolaPro Store.
You can not input a letter in the Next Invoice Number field in Company Options. The reason is a letter beginning an invoice number won't get incremented properly because it is not a recognized number. To a computer 1 + RL1000 = 1 because it figures R is not a number and the whole thing (RL1000) should be treated as 0.