My postal savings credit card bill date is the 26th and the repayment date is the 20th or 11th. If I make a purchase on the 11th, I have to wait until the day of the next month to pay it back. How are these working days calculated?

If the bill date is the 26th, the final repayment date will be 20 days after the bill date, that is, the 16th of the following month will be the final repayment day. All the debt on the bill must be paid off (or apply for installment repayment). Your purchase on the 11th will appear on the bill on the 26th of the month, and you must repay it on the 16th of the following month.