Sales Tax not Contractor's Bid, but requested at end of job

[Adam Gagne]

Hello,

I've received a few bids from Contractors to rehab a property, and most do not include sales tax.  How is sales tax usually handled with General Contractors?

The reason I ask, is that I was about to choose one Contractor and I asked him if Sales tax is included and then he replied that it would be calculated at the end and added to the final bill. Something doesn't sit right with me, and it feels like they are trying to get a little bonus at the end.  

Could you all provide some insight on how you usually obtain quotes from Contractors and handle sales tax?

I am in Houston Texas and we have a 8.25% sales tax rate in Harris County.