When someone asks about a "postmark", we generally need to know how they are defining the term. We assume that when someone asks what the postmark date on their letter will be, they are wanting to know what date the post office will put on the outside of the envelope.
A date is applied in two primary ways. First, a date could occur in the postage that is applied to the envelope via a postage meter. This is a machine that prints the postage on the letter in red ink. A postage meter generally always has a date in it. Secondly, a date gets applied by the Postal Service when they "cancel" the stamp. This cancel process occurs on every letter that has a first-class stamp on it.
Generally, there won't be any postmark date on a letter you send through MailmyLetter.net as we use a postal permit, also known as an indicia. If you look at the mail in your mail box tonight you'll notice that many of the letters that you get don't have a stamp or a postage meter image, but instead have a pre-printed postal permit. These normally say something like "First-Class Postage Paid".
If you are not wanting a date printed on your letter, you are in luck as we don't generally put postage on the envelopes in a way that a date stamp is needed. Keep in mind, the USPS knows when we mailed the letters, so lets try to keep our mail honest.
On the other hand, if you really want a date on your letter you could choose the option to have a real stamp applied to the your letter. This method costs a few cents more but will cause the Postal Service to cancel/date stamp your letter.