Go to Settings > Custom fields > +.
Create a custom field for "Products" called "Quantity" and the type should be "Single line text".
Next, for every graduated price, create a product with a unique ID. You can do this by clicking on Add product under Products. Add the quantity to the custom field you just created.
Once you're done, you can search for the product when creating a quotation or invoice.
Clicking on “Search all results” brings you to you a pop-up with an overview of all products - including the custom field in order to be able to differentiate.
Choose the right price and fill in the correct quantity.