Given that your efforts involved a lot of time (and perhaps paying for the party) and afforded the couple a wonderful celebratory evening, along with $900 to apply to their honeymoon expenses, Prudie feels you have given them a grand wedding gift.
You did a lot to give the couple a great wedding and honeymoon. 
mnli hypothesis: You did a lot to give the couple a great wedding and honeymoon.  premise: Given that your efforts involved a lot of time (and perhaps paying for the party) and afforded the couple a wonderful celebratory evening, along with $900 to apply to their honeymoon expenses, Prudie feels you have given them a grand wedding gift.