A few things:
Though newer versions of Access allow you to put calculated fields directly in the table, it is generally not advised. Most robust database programs (like SQL) do not allow this, and if your database ever got so big that you needed to convert it to a better database program, you would run into issues. It is usually best (and recommended) to do all of your calculations at the Query level and not at the Table level.
Also, if all those fields appear in the same record, it is probably indicative of a Database that is not Normalized. While you can work work with databases that are not Normalized, it can make things much harder for yourself done the road. You should probably have separate records types for each line item, so that each record only has a single amount field.
Here is a good article on database design that talks about those type of things: http://sbuweb.tcu.edu/bjones/20263/A...sDB_Design.pdf
All that notwithstanding, in regards to your question, when trying to add fields of which some may be blank or not have a record, you can use the NZ function to handles those blanks/Nulls and convert them to zero so you can do the math, i.e.
Code:
OrderTotal: NZ(Line1Total,0) + NZ(Line2Total,0) + NZ(Line3Total,0) + NZ(Line4Total,0) + NZ(Line5Total,0) + NZ(Shipping,0)