Bring Your Own Detergent
Laundry detergent, fabric softener, dryer sheets, bleach and other basic laundry supplies are often available via vending machine at the laundromat, but it's an unspoken rule that you're supposed to bring your own. (Pro tip: Liquid fabric softener will soften your blankets better than dryer sheets.)
There is no rule that you need to lug those large containers back and forth, though. Lighten your load by filling smaller, laundromat-friendly containers to carry everything you need without feeling like a pack mule. You're also going to save money by purchasing full-size supplies and portioning them out instead of buying the single-use items available at the laundromat. You can also use one of the many recipes available online to make your own detergent to take with you. And remember: You need less than you probably think.
Plus, there's also a potential social benefit for arriving at the laundromat locked and loaded with your own supplies. Regulars may see you as one of their own, a peer who knows the ropes, so you may be treated better for having planned ahead. (But the flip side is that they may be judging you based on your detergent selection.)