reply to my own question, who'd have thought! i just read an older post by someone else (i don't remember her name) asking the same question. so let's see if i understood correctly. the option of prepaying with the TA is not necessary as this amount will be automatically added to our sign and sail card on the cruise. however, if we prefer we can tell them to remove this amount and pay the staff ourselves? we've always paid tips ourselves and after reading the message thread for that post i think i will want to give the tips myself.
please let me know if i have it right, thanks