Use a helper entity in between like 'orderline' as described in many of the example projects and learning paths by Mendix.

Basically you have an Order that's associated to a customer (1 customer - * order) and 1 order - * orderlines. Then each orderline should have an association with the product entity as 1 product - * orderlines. On the orderlines you can maintain the amount of products ordered and the price the customer needs to pay. This allows you to change the price on productlevel.

But it's a bit difficult to create a domain model for you through this forum. I suggest you take a look at the Mendix Advanced learning path in the Mendix Academy as it's project contains an example of a webshop.