where and when do i deduct shipping expenses on 1099-k

Do not enter the 1099-K on the main screen since it is for your business. Delete it. Go to the Income section and enter the business area. Once inside, you can enter the 1099-K business income.

After that, you can enter expenses.

References:

Where do I enter Schedule C?

Where do I enter my self-employment business expenses, like home office, vehicle mileage, and supplies?