Guaranteed Credits are like insurance: you don't pay the insurance premium to Allstate and then submit your claim to Geico. (And like insurance, Guaranteed Credits apply only to accounts with good sending records. If your DVDs have a habit of not being received or of arriving damaged or unplayable, Guaranteed Credits will not be given.)
If you purchase your Delivery Confirmation outside of the SwapaDVD system, you may have paid more (75 cents at the PO), or less (18 cents for e-DC), but you have not paid the slight upcharge for the Quick/Guaranteed Credit and tracking services. Delivery Confirmation can be purchased for $0.18 several places on the Internet.
What SwapaDVD provides, which justifies the additional $0.30 fee, is the service of mapping your DVD across the country, receiving 'quick credit', and being indemnified against possible misdelivery, damage, or loss of your DVD.