First-Class Mail Letter as shipping option

I have this option selected in the USPS plugin but I do not see it appear as a shipping option. I have a product I would like to offer this option for. Any thoughts on how to do this?

Not all options selected in the USPS plugin will always be returned by USPS API.  For First-Class letter, be sure that the total weight of items in cart meets USPS requirements:

Maximum weight for First-Class Mail letters is 3.5 oz; for large First-Class Mail envelopes and parcels, the maximum weight is 13 oz.

If your items are more than 3.5 oz, then select just "First Class"  (not ... Letter)

Thank  you. It seems I also needed to enable "First-Class". And for sure my items needed to have size and weight information set up too.

Here's another question.. and I know this can be difficult. Can I have different shipping options available per product? I have a product for which I wish to enable First-Class Mail Letter, but I don't want that option available for my other products. Any info greatly appreciated.

RE: "different shipping options available per product"
No, it's not possible without major customization.  An Order only can have only one shipping method.

That said, it is possible with a plugin like Shipping Director to use its Packing feature, make separate calls to USPS to get rates per product 'types', and it will combine the rates to show the customer only one method / rate.  (And optionally, it can also describe in the shipping method description the multiple methods.)