USPS shipping with first-class
USPS shipping link with first-class available.
The UI is much better than that of USPS, and strangely, the price is lower than USPS.
It is easy to get refund if the there is an issue with the purchased label (e.g. unable to print it). PayPal sends an email for each label, and it has a link for voiding the label within 48 hours.
This rates from this link is significantly lower than USPS.com.