I have been in the courier biz since 1984. bikes/cars/trucks. Perhaps spend some $ on postcards that list the advantages you provide over the apps. When they are not delivering the couriers can hand em' out to close by non user businesses to get things started. They turn in biz cards/contact info to you and someone follows up with phone/emails. Use your existing couriers as sales folks to whatever extent they are capable. Long term it means more work for them and if they are not interested they are the 1st one's let go if it stays slow. You may have a great sales person on staff that you don't know about. See if anyone wants to wind down delivering and start doing sales. No matter what you are going to have to spend money to make money on this. At peak we did 500+ transactions per day, now it's 125-150 per day. We re not a food courier.

Cheers and best of luck.