What is the difference between mailing First Class and mailing Standard (3rd class)?
The two key differences are delivery time and postage rates:
A First Class mailing is treated as a priority; the normal delivery time is 3–5 business days. A Standard mailing can take up to 2 weeks or longer for delivery. Therefore, you are paying a premium to mail First Class and the postage rates are much higher to mail this way.
The exception is the 4.25 x 6 postcard, as you will receive less costly postage rates mailing First Class than if you mail Standard.
Another thing to keep in mine is that you will receive returns (due to bad addresses/undeliverable mail) on First Class mail but you will not on Standard. For this reason, when you mail Standard, it’s important to consider the quality of your mailing list and how that will affect your postage costs.