On my invitations, whether it's "correct form" or not, I do not care, I just flat out say that they need to let me know if they're coming by a certain day. I've been surprised with people NOT showing up who said they were coming, and I've been surprised with people showing up when they did not RSVP. So now, I'm pretty blunt. On my son's invitation for yesterday's party (which was great, yay!) I filled in the blanks on the store-bought card but then wrote on the top in the blank space "Joseph is having a Star Wars party at our house, and we hope you will join us! To insure that I have enough pizza, cake, and prizes, please respond by Wednesday the 19th. We look forward to seeing you!" and then left my home, cell, and email. Is that what Ms Manners would do? Probably not but oh well. This is the first year that EVERYONE that said they were showing did, and those that did not rsvp (2 children) did not show up. It worked perfectly. I'll be doing it again next year. As for your current dilemma: hmm. Yeah, I'd skip the goody bags probably (if there's not enough), and just dump the stuff in my trick or treat bowl. If you have enough or can stretch it out, cool, but otherwise, scrap it and enjoy the party.
Another thing: instead of goody bags (because I've seen the pain they can be from friends' parties), I buy a cheap thing of bags, leave them empty, and cut a little piece of tape on the bag, and write the name of each child on their bag. Then when they break the pinata, they can put what they gather into their bags. (We do candies, pennies, and small theme-related stickers, tattoos, and toys). Then I do something else instead of toys in the goody bags: this year's Star Wars party: I bought a pack of pre-made certificates and then put 2 Star Wars stickers on each one, filled them out with each child's name on it as a "graduation certificate" for finishing their Jedi Training (an obstacle course we did in the backyard, and fighting our surprise guest, Darth Vadar), and those certificates, they rolled up and put in their bags with the pinata stuff. I also made everyone a lightsaber that would be allowed at the party (didn't want hard plastic ones that would hurt people, or some children didn't have sabers from home)---I saved paper towel tubes, cut them in half, and then took this summer's pool noodle and quartered them at a decent (and easy to handle) length so it'd fit IN the tube, then wrapped the tube in duck tape so it'd looked cooler. Didn't really cost anything, and the kids thought they were cool.