Hello Duckman33 -
It depends on whether you are using calculated shipping or Flat rate for the dimensions and weight requirements in the Shipping Tools section of the lister. If you select calculated shipping those fields are required.
When you select USPS first class package from the Domestic Shipping Services menu, above that you have options for Package size, and the option you would most likely use (depending on the size of the envelope) for the bubble envelopes is Large Envelope (or Package (or thick envelope)) if you are using calculated shipping.
Please note that we cannot offer shipping methods or options that are not available on eBay - and the options available on our lister match those of the Sell Your Item form on eBay - you would see the same options available when using calculated shipping. You should not need to specify the package type for Flat Rate even though you see the options for dimensions and package size and should just be able to select USPS First Class Package.
However, as eBay does not offer a specific shipping method for the bubble envelopes, we are unable to offer such a shipping method via our lister page.