Some might find it unfair indeed. However, it's not unheared for hosting providers to ask for a certain amount for free Let's Encrypt certificates. One might argue the addition of the free LE certificates as a service costs money for the hosting provider, e.g. increased work for the IT staff, questions to the customer relations et cetera.
That said, there are many hosting providers not asking for extra money.
Personally, I'd say every hosting provider should provide free (Let's Encrypt or other free CA) certificates for every user, so asking money specifically for this service should not be done. Instead, a hosting provider should consider increasing the fee for every user making use of this service if necessary at all. I.e.: every shared hosting user. And not making it opt-in and requesting a separate fee.
You can find a table of hosting providers offering Let's Encrypt or not at the Certbot site:
I'm not sure what will happen you attach cloudflare with flexible mode in front of it (it's not really end-to-end, but your client will see the green lock)
if it were hosted on your server how about left it as http and attach a reverse proxy with https in front or it? like caddy or nginx. https://redcart.pl doesn't have selfhost option, so this doesn't apply
keep mind there is another e-shop frontend company , redcart.com, which is unrelated