Goods and services tax

credit card companies charges you for the credit facility provided by them. This charges are income of credit card companies and they levy GST on their charges.
Whereas purchases that you make , GST is charged by the vendor for purchasing goods from them.
Note that credit card companies won't charge you GST for purchase you make, they charge GST for their income I.e. credit card charges

The GST is levied over services provided by the card provider, including interest levied over the card etc. But not over your expenses/purchases.

thanks for lucid reply.