Helvetica font is not embedded in at least the customs forms (commercial invoice) generated by ShipStation. This document is needed for international shipments, so it is quite important that this PDF is generated properly. In its current format, the PDF does not print correctly, nor render correctly in external PDF editors, so it is impossible to print with the information required for international shipments. Sometimes it works, but it should always work.
The only workaround I've found is to convert the file to an image format like JPG and print the JPG file. This results in lower quality and inability to make any edits, in case additional information needs to be added, which is essential.
On a related side note, more customizability around these files like the customs forms, commercial invoice, etc. would be a really nice feature for international shipping. Right now it's not possible to make sure the necessary information will be included in ShipStation's auto-generated files.