In the recent years issuers have been enforcing this, as the market place has become more competitive. However if you call the your financial institution and request to remove the charge, sometimes they comply. If you make it a habit they probably wont remove the charges. Otherwise it is worth a try. I tried this four or five times.
Some credit cards charge an annual fee for using the credit cards.
The interest is calculated using compound interest instead of simple interest. It is the current credit available to you. It is calculated by adding the outstanding balance on each day in the billing period, and dividing that total by the number of days in the billing period. The calculation includes new purchases and payments. So the day of payment and day of purchase affects the finance charge.