Swiss Post is very thorough about the quality of their shipping labels and often check to ensure that the quality of the postage in their network is good enough. This is why you need to have your account signed off after connecting via Shiptheory before you can start sending shipments with them.
You may receive an email from Swiss Post's PostMail Acceptance team regarding your shipping labels if they have determined that the print quality or label alignment (etc ...) is not completely correct.
This is nothing to worry about and normally just requires a small and adjustment to your printer's settings.
You can rest assured that the shipping label PDF you are provided to print via Shiptheory is 100% correct and is produced by Swiss Post's own shipping API.
If you have any further questions or concerns about this, please contact our support team who will be happy to help.