Transfers require heat and pressure to correctly bond with the fibers of the cloth. If the bond is poor, then the image is basically melted to the top of the shirt and will not survive washing.
So, find the hottest iron you can (often measured in watts) and use as much pressure as you can.
That said, follow the instructions that come from the transfer paper manufacturer.
It is also possible that your local library checks out tools (such as a t-shirt press) to the community. You might also check the local community college or high schools. Sometimes sports teams make their own shirts or add names to jerseys. Their coaches might be happy to get some cash.
Finally, if you are making that many shirts it might not be crazy to buy a press, use it, and resell it here.