Thomson (TUI Marrela) have a strict no discount policy on their cruises. Whereas the big boys like Royal Caribbean, Celebrity, MSC etc allow agents to sell their cruises and as a consequence, the agents are desperate to sell the cabins, hence they undercut each other. I usually use IGLU, Cruise118 and Travelbag to compete against each other and usually get the holiday far cheaper than the advertised price. Unfortunately Marella are different, When I booked with Marrela 5 years ago (Thomson then), no travel agent would advertise their cruises so booking was done direct and with no discounts. I am going on Marrela again in 22 days time and this time when booking I see that IGLU are now offering Marella cruises. So, I phoned IGLU up to compare their price with Marella direct and was told that the best they could do is knock a tenner off. Sadly, there are no discounts or freebies with Marella.
Tui Marella prices fluctuate on line I would only book a long time in advance if it was to get the specific dates and cruise. Normaly i keep an eye on the internet price and go for it if there is a big drop. The shops will match the price but wont undercut they may charge an admin fee but will then print any info and tickets for you my local Tui shop does so best to enquire.