How to add a product

[Andrew_Stock1]

Hello everyone, I am attempting to build an app that helps sales consultants calculate the GP$ & GP% in each sale for my Mattress Retail Shops.

Often customers will choose a mattress and then a base. To select product quickly the app works with a series of drop downs for example;
Choose Product (From a list of product categories i.e. ‘Mattress, Base, Adjustable Base, Pillow’ etc.)
Choose Size (From a list of Sizes, Single, Double, Queen, King etc.)
Choose Product (From a list of the various mattress models)

Now here is my question: Having chosen a mattress, how do I provide the option to choose or add another product such as a base or pillows? And then ensure that the cost of the new item/s are added to the Sales price?