We never pre-book in case we get the trots / change our minds when in resort. If you're not thru your own tour rep, will you get all your money back?
There are plenty of local vendors & all hotels offer trips (altho they sub-contract thru local vendors)
Once in resort, you can haggle the price with them face to face.
If you put the names of the firms into the forum search engine (above all tpoic listings, nr horizontal green strip) you will find previous discussions on some of them.