At the moment there is a bug known to Extension Works relating to First Class Mail.
First Class mail is associated with letter mail and also Parcel mail.
It has the same service ID for both methods.
If you do not have First Class Mail selected in your Parcel Mail Services it may be available for your customers to choose at the checkout.
The way to compensate this from happening, is to select an option from the Envelopes for letter Mail setting in the admin area, you could select Priority Mail Flat Rate Envelope.
The same applies for International orders. Choosing a Priority Flat Rate International Envelope will solve that as well.
This will not allow the Service to appear as a Parcel Mail option.