Definitely make an enquiry with the seller as Bad Angel says.
The postage shows free shipping for each additional item but you bought from two separate listings.
If for example you had bought two of the blue one then the invoice would come through with only one lot of postage on it.
You have bought one pink and one blue and it does seem to me that the seller has already combined and is charging you just $5 for the second item and $25 total instead of the $40 that they could have charged you.
