What an old thread.
Open eyes, while I understand you have a fear of needles, taking a snake to a pet store where there are animals coming and going all the time with no quarantine is not likely the best circumstances to take him to for an injection. You literally have no idea what is running around in the pet store, what sort of viruses, bacteria and fungi are being passed back and forth because of the nature of pet stores.
You can't control what is passed on to your snake or yourself while in a pet store, and it really breaks all quarantine procedures to take him there repeatedly.
If you have trouble giving the injections yourself, better off asking the vet that subscribed the anti-biotics to you to do it for you to avoid your snake catching worse things at the pet store.
Baytril is an excellent medication, but as always, a Vet goes to school for 8+ years to learn why and when any medication should be given and for how long. Never use the internet to diagnose or treat for anything beyond simple first aid.