do the sum of two commerce_price field with programming api
There is no basic price addition function, though we have API enhancements on the roadmap for Commerce 2.x. I'd take a look at what commerce_order_calculate_total() is doing to figure the order total price field; you have to sum the amounts, like you've already discovered, and you then use commerce_price_components_combine() to merge the two price fields' components arrays together. Hope that helps!