I have t but I do drop off all my shirts at a local ironing business. They are very very popular. I pay 95p per shirt and drop the shirts off with hangers. I collect the next day and the shirts are beautifully professionally ironed and hung and covered in plastic (like dry cleaning). This business charges per item rather than per bag.
I would think if you are being paid per black bag full it's going to be very hard work for not much return as I assume the company take their cut.
Can't you get some prices and advertise locally? Cut out the middle man.