What if I don't want to sideload, but some app I really want to use it says screw appstore and sideloading is the only way? For example to pay for parking or us

What if I don't want to sideload, but some app I really want to use it says screw appstore and sideloading is the only way? For example to pay for parking or use a loyalty scheme in some shop. Of course, technically I could use other means or avoid such businesses altogether.