Okay, I need to charge my Michigan customers 6 percent sales tax, including the shipping. How do I do that on Yahoo? What do you all do?

I have always charged Texas sales tax to Texas customers-and paid the state everything I collect. It would be silly not to, there's a record of it out there online!
I just add that to my EOA letter, along with the winning bid and postage costs, insurance options, etc-'Texas residents, please add 81/4% sales tax to your pre-postage total'. Thing is, you'll be surprised how many people don't pay it! If it's PP, I just refund their money and ask them to resend it with the sales tax.
Hope this helps!

Texas-Treasure is right...

to add to it - if your PayPal settings are intact, when you invoice a person In-State, the tax should be auto

...but alas, Yahoo is unlike eBay (and most other alternative sites) whereas they do not have a check box feature for ensuring Sales Tax is charged to the appropriate state customer

Marty