URSSAF charging the wrong rate?

My auto-entrepreneur business is migrating from liberale to sales. Previously all my work was consultancy but in September last year I started selling so added ‘vente de merchandise’ to my activities. I duly received the new certificate d’insription of this from INSEE. However, when the declaration form for recettes arrived it still only had a column for CIPAV activities at 18.3%. I went in to the URSAFF office and they confirmed I should declare 12% on sales, even hand wrote 12% in the next column down. I duly sent in the return, but then received an ‘Avis Amiable’ with a demand for another 6.3% on this amount plus a fine for late payment.

Back to URSSAF, who tried to brush me off and send me around the houses as usual. I stood firm but they kept saying I could only have one rate and would either have to drop any consultancy work or pay 18.3 % on everything. Eventually they agreed to conatct INSEE to ask if two different rates was possible, that was two weeks ago and so far no response. Obviously there are plenty of people offering services and sales, but have you seen anyone else with a ‘liberale’ profession and sales, at one point I bluffed that I knew somebody who was and they immediately asked for details to help my case.

The added complication is that my consultancy work is usually in the summer/autumn so it now looks like my principle activity is sales, which means they may want to move me to RSI from CIPAV. Any ideas or thoughts on this would be useful.