The US Postal Service First Class Mail option is only available for orders weighing under 14 ounces in weight. If you have items that are under 14 ounces and you don't have the US Postal Service First Class Mail option disabled in SEO-Cart, then the US Postal Service First Class Mail option should be displayed to your customers.

If your US Postal Service First Class Mail option shows "Postcard" in the name and you want this to be changed. Please read the following:

This "Postcard" method is returned when you have very small weight for a product and if there are no dimensions set for the package. Since you can only do "First Class" for orders under 14 ounces, you can modify these low weight products to be a little more weight. Increasing the product weight, while still leaving it under 14 ounces, should change the word “Postcard” to “Package” in the returned Postal Service First Class method.

Another option (instead of increasing the weight a little) is to set quantity level package dimensions for your low weight products. You can do this by modifying your low weight products (under 14 ounces) in SEO-Cart and by clicking the Add Bulk Quantity Packaging Definition button under the "Shipping Options" section. Here you can set the Quantity Start to 1 and enter package dimensions for the item. Doing this should also remove “Postcard” from the Postal Service First Class method and replace it with “Package”.