Over payment of URSSAF cotisations; how to effect a correction

My wife has been a micro-entrepreneur since late 2018, selling items online (through Etsy).  For most of 2019 she was declaring her income based on the amounts reported to her by Etsy, and paying the cotisations percentage based on that.  Toward the end of 2019 she realized that the amounts that Etsy were reporting to her included the sales tax that they were collecting, but the sales tax was not part of what she received and thus did not actually contribute to her income.  For this reason her declared income, and thus the cotisation payments, were overstated for the first three quarters of 2019.

1. What is an appropriate way to make an adjustment to correct the over payments?  Can she simply declare an appropriately smaller amount for the fourth quarter than she actually received, thus correcting the overall amount for the year?  This would not result in a declaration that is significantly out of line with the others for the year.

2. If the above can be done, does it make a difference whether or not it’s done in the year in which the over payments were made?  I.e. is it best to make sure that it’s part of the fourth quarter declaration, or is it acceptable for it to be (partially or completely) corrected in 2020?

3. If adjusting the fourth quarter amount down from the actual received income is not an appropriate way to correct it, what would you recommend?