I always make the buyer pay shipping. I much prefer to use Mercari's labels.

I don't like to use Paypal.com/shipnow or Pirateship and ship on my own because I don’t like to pay fees on the shipping.

If you make the buyer pay shipping, you only pay fees on the item price. I also like the convenience of the labels.

I use USPS First Class or USPS Priority if over a pound. For both options you can schedule a free usps pickup.

Sometimes if over 3 pounds I might select UPS or FedEx, depending on whatever is cheaper, only because it's significantly cheaper for the buyer. If I select FedEx or UPS, I tend to mention in the description that I can ship via USPS if contacted before purchasing. [Hawaii, Alaska and PO Box buyers need USPS.]