Unless the fund you want to buy is not sold in the postal savings office, consider changing banks.
If you change banks, you only need to apply for a card in that bank and then open an account, which is also very convenient. In other words, the commission on consignment is more expensive than direct sales. I suggest you consider which fund and buy it directly on the website of the fund company. The commission is cheap, so you don't have to go to the bank to fill out the bill.