Hi Anon! I sell through my online storefront on Shopify so when customers make a purchase, their address information is already processed into a shipping label for me to just buy and print! Sometimes if they choose a courier method like UPS or DHL I'll have to print an extra few pages to attach to the parcel (customs declaration form).

Buying prepaid labels through third parties like Etsy, Shopify, Storenvy, Bigcartel or even Paypal is often times a little bit cheaper than going in person to a post office. I also know of a site called Pirateship that offers the same type of service (I believe this is also Paypal? But it's been a while since I used it so I'm not sure!) though this site is also a monthly paid service :(

I'm not sure if this is the kind of info you needed, feel free to email or DM me if you need more help!