I have to order a bunch of flowers for an upcoming cake.
I was thinking to just charge double what i paid (+s&h)
does this seem reasonable.

The advice I used most was to charge what a florist charges for fresh flowers.
Are you selling the flowers separately?
If you are not making any flowers but sub'ing the purchsaed one I would not charge extra.....it should be included in the total price.
As an example, if I used $10 worth of purchased gp flowers (my choice to use rather than make them) I would not charge dbl what I pd for them....that's part of doing business.
Depending on the flower I can make them from b'cream, fondant or gp. If I choose to buy to save myself time why should you charge the customer? Figure out what it is going to cost you to make the cake and that's what you charge.......not for dbl the price of each flower.
Anyway, that's my feelings.