For my consulting service company, I am reimbursed for contract travel meals at per diem rates. How do I categorize the meal and the overage. For example, if on a trip my total per diem is $100, but I spent $120 on meals. Therefore, the client, reimburses me $100. How do I categorize the $100 and the $20 that is over per diem. Do I attribute the $20 as Direct costs and therefore a part of COGS or an expense?

I think you could make an argument either way, but I'd include it as part of Cost of Services.

If you are in the US, I'm wondering if you are aware of the IRS rules regarding per diem. If you are actually using $100 as your per diem rate for meals, that exceeds the IRS allowable. Are you tracking all of your meal costs in a separate account for tax purposes?

Publication 1542 (10/2010), Per Diem Rates

If you need help on these rules, I'd suggest posting a question on the Tax Board. The tax experts there probably deal with this often and will be better able to help you. Thanks.

https://www.askmehelpdesk.com/taxes/