Hi @troyw88 - welcome to the Envato forums!
To clarify the link above, the “series” (1 year / 52 episodes) coverage is specific to a Standard Licence purchased through Envato Market (for example, a track from AudioJungle or a video opener from VideoHive).
For items licensed via an Envato Elements subscription, things work a bit differently. There is no “series” equivalent there, so you’d need to register a licence for each video separately. As it’s an unlimited subscription there are no additional costs, but that ensures that the authors are credited (and paid!) appropriately for the use of their items.
For your question, if you are creating the video for a client, and just giving them the option of which intro and outro music is used, you should be fine. You can’t re-sell individual assets (e.g. redistributing just the audio track), but you can produce and sell a video that uses those assets.
See Section 5 in the Elements license:
You can use an Item to create an End Product for yourself or for a client. If you use an Item to create an End Product for a client, then you can transfer the End Product to your client as long as you have followed the requirements in clause 3 and your subscription remains active.
Each video that you produce is a single End Product, and you can transfer that to your client as long as you have an active Elements subscription.
However, I’m not sure how the licence would cover that client going on to sell the videos to customers themselves.
I recommend opening a support ticket with Envato Elements Help and Support - that way you can get a definitive answer from our license experts, specific to the way you and your clients intend to use the videos.