Hi,
I doubt there are any OTC medications that could help.
You could have your local vet do a phone consult with a qualified reptile vet possibly? That way you get expert advice locally for just a little more cash.
Have a look for semi-local vets on these three sites;
Arav.
Herpvet.
Anapsid.
As to the seller and shipping I'm not sure you would normally get the shipping refunded - I have heard of people getting the animal cost refunded but not the shipping.
The sack and crumpled newspaper are standard enough but it is usually in a poly insulated box with good heatpads and shipped next day.
If you had to do a payment plan I'm guessing this isn't a cheap snake we are talking about so you would expect good shipping.
You might try asking for help with the vet bills instead of a refund if you like the animal.
But I think you are going about it the right way in giving the seller an opportunity to make it right first.
And, if you do go to the BOI remember to have all the information such as reciepts, dates, emails etc ready to post to give everyone a clear and accurate timeline and document trail.
And if you manage to go to the vet make sure and get a reciept saying what is wrong with the animal etc and have that all scanned and ready to post up too.
dr del