Does your state have a sales tax? If so then you probably are required to collect sales tax on the invoices you send to your clients within your state (at least). Some states don't require tax on work done for clients outside the state, but that is a state-by-state thing.
It can get ugly if you "have a presence" in several states. In that case you have to collect the local tax in the states that you have an office on the work done for clients in that state. In the case of multiple offices in multiple states/counties, you'd better already have an accountant and you shouldn't be asking a bunch of engineers.
Once you collect the sales tax, you then have to remit it to the state monthly or someone goes to jail. States are really cranky about that.
Another thing they are cranky about is they want their tax whether you collected it or not. If you fail to collect it, you are still liable for it. It can get complex to determine if the $100 check you got is actually a taxable $100 or $93.45 plus $6.55 tax. In the first case you owe the state $7.00 (if the tax is 7% like it is here) in the second you owe $6.55. You can decide how you want to do it, but then you must do it consistently from then on.
I just finished my 2007 taxes and found that the 1099's that I got from companies that I charged sales tax had included the tax amount in the total reported to the IRS. This just means that you have to make sure that you claim the sales tax submitted to the state as a legitimate business expense. It felt really odd to deduct a passthru expense, but that is the way it works.
David Simpson, PE
MuleShoe Engineering
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