If you are based in the US, you may see a mark on the envelope when you receive your sample or see an envelope from one of your recipients, These are postmarks, and are standard practice within the USPS. This is important to bear in mind if you want your letters to appear as if they were written and posted locally. You can read more about postmarks here:
One alternative to ensure your postmark is local is you can request your notes as a bulk shipping order. We can send your notes to you in bulk, pre-stamped and ready to send, so you can then post them from your city so the postmark will be a local one.