Outside of a monopsonistic labor market, how in the hell do you pay wages that are "below market level" without having turnover rates >100%?
Market level wages are determined when the employees have a reasonable expectation of organizing to obtain higher wages. The Ground system has gotten around that system by taking away ANY ability for the drivers to organize. Just the real threat of unionization makes employers pay a "market wage", instead of the lowest they can possibly get away with.
A market wage ISN'T the wage that those with no power are willing to accept (how many Express Couriers would work for Ground wages.... there is your answer), a market wage is the the wage that those who participate in an equitable labor market accept - that includes a reasonable option to organize to bargain with their employer to gain what THEY feel is equitable compensation for the work they perform - IF THEY SO CHOOSE.
The wages Express pays, ARE what I'd consider "market level wages". They are definitely BOTTOM of the mark, but the Couriers and RTDs have so far refused to organize.
THE ONLY REASON EXPRESS IS PAID WHAT THEY ARE AND GROUND IS PAID WHAT THEY ARE, IS THE POTENTIAL OF EXPRESS TO UNIONIZE. That is the SOLE reason there is a differential in compensation between the two groups of employees doing the exact equivalent work.
The helpers who drive for Ground are those who are so far down on their luck, that they are desperate for work. If they worked for their real employer, they'd unionize in a heartbeat and their wages would rise. The situation that exists in Ground is legalized exploitation of those who have no power to change their situation with their employer - since they aren't "technically" working for their true employer.