Makes no sense

Retainers costs $2 each. So why is it double the costs?

I have 4 extra retainers. That's 2x4=8 by my math. If i sub for 30days. Subtotal is $15. Their must be hidden tax or something we all don't know about, since their is $7 added on that. I like to know for what that $7 is from. Taxes?

Granted it's not much. It does add up and SE is already making a killing from us all.

A subscription is $14.99, I have 7 extra retainers, which is an addition $14... my total is $28.99 for a month. There is no hidden cost or taxes, at least on my end.