Is it possible to pad USPS shipping rate for a single specific shipping method?

[zippygaloo]

I only use USPS Priority Mail and USPS First Class Mail Parcel. With Priority mail the box or envelope mailer are free. With First Class Mail Parcel, I have to provide the box or envelope mailer (which costs money). So...

Is it possible to pad USPS shipping rate for a single specific shipping method?

Thanks