Hi Caryn,
Are the seeds you collected hard? If they are hard, they are good. If they are soft, they were not pollinated, you picked them too soon, or you stored them improperly.
There is nothing wrong with storing them in an envelope! You don't want to put fresh seed in plastic unless completely dried. The excess moisture will have no where to go and the seeds will go bad.(There are a few seeds that must be kept moist and sown immediately to grow like citrus and some woodland plants, but most seeds need to be dry.)Keep your envelopes of seeds in a cool,dry, and dark spot, like a closet in a cooler room in your house, to make them last a few years.
Since it is a tropical type plant, you need to plant the seeds and use bottom heat to germinate.
Hope this helps,
Remy