First of all, the current patent acceptance office can handle more business. In the past, it could only handle ordinary applications, but now it can also receive intermediate documents, so you can submit the corrected documents to the acceptance office.
However, this change was only implemented on February 1 this year, and some agents may not accept it, so you can ask, or mail it directly to the Patent Office in Beijing.
Secondly, the deadline for you to mail the documents is the issuance date of the supplementary notice + 15 days + 2 months. The final date is based on the postmark date of the document you sent, not the date of receipt by the other party.
Finally, there is no charge for corrections.