If you go to directly to a shop which is 10 miles away, and then go directly home, you indeed entitled to claim 20 miles. If you go from Shop A to Shop B and that trip is 5 miles, and then from Shop B directly home for 12 miles, then you claim 10 + 5 + 12 = 27 miles.
I assume that you decided that the standard deduction for mileage is best (TurboTax would have prompted you with help to figure that out). In which case the calculation of 1000 x $0.55 = $550 is correct. TurboTax will enter it onto Schedule C for you. If you paid for tolls or parking, that can be claimed in addition.
Don't forget other business deductions such as paper, pens, copies, printer ink, and possibly a home office deduction if you used a room exclusively for business.
Shopping Southeast Pennsylvania, Delaware above the canal, and South Jersey since 2008