I have huge problem which I can't handle with. I'br read many articles about fees and still I don't know how can I solve my problem.

Maybe I will show you in an example:

1. user A creates a product in his shop (for 0.10 bitcoin)
2. user B creates a widget of this product from user A
3. user C buy this product from the widget which belongs to user B

so user A and B should earn money therefore I must split sent them btc.

product price: 0.10 btc user A, gets 70% of 0.10 btc user B, gets 15% of 0.10 btc and I get the rest, 15% of 0.10 btc

everything is okey, but now I have make 2 transactions from my wallet: 1. send btc to user A 2. send btc to user B

and here it is the question: how can I calculate fee for sending money to user A and B?

I'm using coinbase.com as an API to sending / getting btc.