I'm not aware of any studies that measure the effectiveness of amoxicillin specifically in ball pythons. I've treated many ball pythons with upper respiratory infections and I myself wouldn't try it. There are more than enough anti-biotics available that actually have been tested on and studied specifically in ball pythons so that the proper dosage and length of treatment are documented.

The particular strain of bacteria causing the infection could have a tolerance or may just not be affected at all by amoxicillin. Your best bet is to take your animal to a vet and have it cultured. That way, the vet can prescribe the most effective medicine available for treating your snakes infection.

And for what it's worth, injecting medication into a snakes food is an ineffective delivery method.

Unless you really know what you're doing, your snakes life isn't something that you should experiment with.

-adam