Double charges or N00B?

[cadaverworld]

Hi, I am new here and hoping I am missing something obvious.

A few time now, I have gone to create an order with the description saying something like “Just $20 for the addition/ upgrade of 10 items, + 1 for the original”.
or
"Buy 10 for $20.“
I look at the field to click, it says $26.58”.
Assuming this means the original $6.58 as well, I click it and find it then says I am to be charged $33.16.
This means the $6.58 is being charged twice.

I can not see how to create the order, without that happening.

Can someone please explain to me what I am doing wrong or if there is a reason I missed?
Is the seller accidentally putting the incorrect price in the description? It seems to happen a lot.

Thank you.

[cadaverworld]

Please delete the original post, and reply. Thank you.