Thank you for your post.
I don't know how your tables are set up, but it appears that you only have the product/service field as the key field to obtain the price. You may want to look at your relationships graph and also add the size/dimension field to the relationship. That way, both fields are necessary for the lookup.
I'm not sure of the relationship of Service_priceNNA. Assuming this is in the other table, perhaps you want to check to make sure either of the key fields are blank before using it. That is:
If (IsEmpty (ServiceNNA) or IsEmpty (Service Size Desc NNA); 0; ServiceNNA & Service Size Desc NNA)
That is, if either ServiceNNA or Service Size Desc NNA is empty/blank, then there is no value. Once there is a value in BOTH fields, then we concatenate them together.
Does that make sense?
Let me know if you need clarification for any of the above steps.