Retail shipping price is how much you charge your customer for shipping. On the other hand, the Printful shipping price is what we charge you for shipping when we send out your order.
This means that the amount you pay is:
Printful product price + Printful shipping price
And the price your customer pays is:
Your product retail price + Retail shipping price
You get to choose the retail price and the retail shipping price. For example, you can make it equal to the Printful shipping price so that your customer is covering the cost, or you can set it to $0 to offer free shipping.