The basic idea is that you add the personal banner or personalized party supplies (such as bubbles, lip balm, stickers, etc) to your list of services. Set a price that will (1) cover the total cost of the item(s) including taxes and shipping, and (2) includes a little extra money for your efforts. 😉
The awesome part is that you can offer almost anything that is available on the site, and you won't have to keep any extra product on hand or spend your own money in advanced. Just make sure that you allow for delivery time. Also, I strongly recommend collecting an upfront deposit when offering this service so that you won't have to come out of pocket.
Final tip...if you offer this personalization as a new service and your customer asks if you make these items yourself, you can be open with them by saying that you use a vendor. You shouldn't feel obligated to share the name of this vendor as that would defeat the purpose because your customer could then just go directly to the source without you.
I hope you found this little tidbit helpful. If so, please let me know via email or through the Sweet Success Project Facebook group. I would love to hear your thoughts!