Why do you have to have a small receipt to issue a health certificate invoice?

I need a receipt, because I need it to get a health certificate. Finally, this receipt will be taken away. According to the relevant information inquired, there is a formal receipt invoice for this fee. If it is free, there is no receipt and invoice. Just get your ID card. After the physical examination, you will get a receipt. This is the receipt. You will get a health certificate and receipt. Finally, this receipt will be taken away.