Add product options......
I hold a lot of t-shirt stock. One model number from one brand can have up to 60 colours and up to 10 sizes - that's one product with 600 colour and size combinations, and I can supply up to 100 products.
Not having these product options to select on each sale gives me no idea of what my current stock levels are on any given colour or size and doesn't allow me to create a detailed invoice for my customer.
This effectively will make Invoice Expert utterly pointless for anyone dealing in products that have multiple variations.
Even if I could have a simple text box in the row when I add a new item to a customer's quote or order, I could manually enter the colour and size. It wouldn't help me with inventory tracking but at least I could give the customer a detailed invoice and review previous orders and know what they'd bought.
Great product, but this is a pretty fundamental flaw - I may have to ditch it.

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Chris commented
John;
Enter a different product number for each brand , colour , size then enter the inventory quantity. This way when you make the invoice you choose the correct product number and it shows the invoice and is deducted from the inventory to help with inventory tracking.Looking at the date you might have already figured this out. :) -
David commented
We have a similar problem although its about the colour and fabrics used on sofas.
One design with multiple choices of colour and fabric.
At this point it does not present a problem as we are new but going forward it would be a great advantage -
John commented
*1000 products that is