Bill not given with purchase

I made a purchase with a company. They did not send me any bill with it, except for a cash memo that just mentions the name of the distributor.

When asked to issue a bill against an extra 18% of the amount, they are not even providing me the details of the party who will issue the bill. 

Can they do this? Mention that for a GST bill, extra 18% needs to be paid? Because, they must not make any sale without a bill, right??