Seems like you have a real structural issue here. Why are you putting the costs in a calc?
"IF" statements require a false option. Either change your IF to CASE, or add in a false option for IF.
(If ( Audio Visual = "2000 LM Projector" ; 360; 0)) + (If ( Audio Visual = "DVD Player" ; 50; 0)) + (If ( Audio Visual = "8' Screen & Skirt" ; 75; 0))
Each IF statement needed the "; 0". So for the first Calc it will return 360 when true, but it will return 0 when false.
Having said that I agree with mr_vodka in that this approach seems strange.
"IF" statements require a false option.
I cant say that I agree with that statement.
I took the OP's issues as having values with a checkbox, in which case using PatternCount or FilterValues would be used. However, I still stand by my issues that I have as stated in my orig post.
I agree with Mr. Vodka. If you use the Advanced link to search this forum, using "invoice" as your search text, you'll find a number of relevant threads on the subject.
You probably should be listing these items in a portal with one item in each related record.