just embed youtube video on cart page template
whats the store address?
also note, if embedding direct youtube video, it will show related videos at end...
I can give you cost once I see website or DM me...
How large is the video? Less than 20mb?
1) embed youtube (if over 20mb)
2) upload mp4 within Shopify then use code to display video (recommended if smaller than 20mb but recommended up to 5mb for speed)
3) upload mp4 to another host website (ie: a-domain-you-have-hosted-with-another-provider.com then reference the URL and use the resources and bandwidth from the other domain for the video)