Which category do I use for monthly fees for shipping services (stamps.com) ?

Welcome to the Community forum, @littleloveys.

Each business has a unique structure, and it's the same with the accounts used when tracking transactions. That said, I suggest consulting an Accountant or a Tax professional. They can help you select for the specific category.

Moreover, I don’t want to leave you empty-handed, so I’m adding some articles that will help you in managing your account:

• Categorize and edit transactions in QuickBooks Self-Employed
• Categories in QuickBooks Self-Employed
• QuickBooks Self-Employed Schedule C Categories breakdown

I'm always here for you if you have additional questions, just leave a comment below. Have a wonderful day ahead!