Yes, Interflora are not at all transparent about their pricing strategy I'm afraid.
Given how hard they make it for their members to make the product look like the picture, they also have a "no quibble" refund policy. So if the flowers don't look like the picture ( which they never will) you have every right to complain. And keep the flowers or have a replacement or a refund. The florist bears the full brunt of that cost.
It's an awful system and I'm still surprised that do many florists offer it, given the poor margins for them. I don't know a single florist shop that offers Interflora that sells good quality flowers, either. They tend to buy cheaper/A2 or B2 grade flowers to try and bump up their profits.