There are many printers on the market that will be suitable for printing shipping labels.
They need to be label to print labels that are 100mm wide, and 150mm tall (4x6").
Ideally their print resolution would be 300DPI (dots per inch), although 203DPI are commonly found and should be ok as well.
When considering which A6/thermal printer to buy it is worth looking at the label cost. In particular, the Dymo 5XL printer has very expensive media, so is not commonly chosen these days.
Other printers on the market can use generic labels that are much cheaper to buy.