USPS First Class Envelope - Not an option?

Hello all!

I am needing to use the USPS first class mail option to send out our products to people. We do decals, so we usually just send everything out in an envelope, which doesn't weigh very much and is not in a very large package. So...it would be ideal to use the First Class Mail Envelope option in the settings for USPS, however it is checked but is not showing up as an option on the checkout page. Priority and Express show without a single problem, so I'm just wondering what I might need to change in the settings?

I read that there were problems when USPS and UPS were enabled at the same time. This is not the case for me...UPS is not enabled at all.
I have been placed on the USPS production server, so that is not an issue either.

My current settings are as follows:

Domestic Services: (that are checked)
First Class Mail Package
First Class Mail Large Envelope
First Class Mail Letter
Priority Mail
Express Mail

Size: Regular
Container: Rectangular
Machinable: Yes
Dimensions (LxWxH): 5 5 5
Girth: .125
Display Delivery Time: Yes
Display Delivery Weight: No
Weight Class: Ounce
Geo Zone: USA
Status: Enabled

I am running Opencart 1.5.1.3

Thank You!

envelope rates only show if the weight is less than 3.5oz
and I believe the overall girth has to be smaller than 5x5x5
Try 1x1x1

Amazing!

Other people had mentioned that it should show if it's less than 10 ounces, so I thought I was good there. But nobody said to change the overall girth from 5 5 5 to 1 1 1. That worked!

Thank you for the input!