If you are just selling, and you can provide sales certificate to say that you do not understand that the product is a patented product, you will have stopped selling it immediately after receiving the notice. So although you have infringed the patent, the patent law stipulates that in your case, the patentee cannot demand compensation from you and does not need to provide proof of sales before the application date.
Article 70 of the "Patent Law" uses, offers for sale or sells for the purpose of production and business operations a patent-infringing product that is not known to be manufactured and sold without the permission of the patentee, and the legal source of the product can be proven , shall not be liable for compensation.
There is no requirement to provide proof of sales before the application date. Even if he authorizes you to start selling, as long as you can prove that the product is from a legal source, it is an infringement, but no compensation will be paid.