What are the alternatives to BarTender?

Option 1: NiceLabel. This is similar to BarTender and cost similar.

Option 2: Free software that comes with the printers

Most printers come with some software to print. You can use the same, but it won't be automated like BarTender or NiceLabel. You can enable the "Download Labels Print File" option in OpenSpecimen "Settings" and manually copy/paste the labels to the printer's software.