We ship thousands of packages a month and the discounted rates/incentives UPS gave us is dismal. We use ShippingEasy and their rates are great, which I'd assume are the same or comparable to PayPal, Shipstation, etc...
Signing up for a third party shipping partner like that is well worth it.
I will say to be cautious with UPS. Getting a them to pay a claim for damage is impossible. I also had an employee dedicated one day a week to filing claims for lost packages. USPS is typically a shitshow, but this year they outperformed UPS by a LOT. We only use UPS now expedited services and heavier (3+ pounds) packages.