From what I was told (and I may remember this wrong), they pay something like 7p per item delivered. Delivery is the bit that costs, not picking the stuff up. Or rather, TNT etc. only pick up from large firms so do it in one hit, rather than trawling postboxes. So they take maybe 75% of the costs for that pickup, spending back out 25% on the actual delivery. Meanwhile RM pick up all the small lots to deliver which cost a lot more.
So effectively whoever forced RM to accept the piss poor rates they have to charge the other companies, is the one who's fooked over RM.