Luxplus Free Membership Turned Hidden Fee

I purchased several deodorants from a website called Luxplus. Membership was required to make purchases, but it was free. After becoming a member, I paid for the products and completed the transaction. The products I ordered have been delivered, and I have settled my account with the site. Additionally, I have canceled my membership. However, they are now demanding extra money from me. What's the point of offering discounts if you're going to charge a membership fee? To make matters worse, they have shamelessly involved a collection agency. I will never shop from Luxplus.nl again. If they continue to harass me, I will file a complaint.