Express Route

Hi All,
I have a client that we are migrating to O365.
There is a need to implement "Express Route" https://docs.microsoft.com/…/expr…/expressroute-introduction

Now that is easy to achieve on MS side.

We want to get better performance on ISP side as well and asked them (The ISP) to create a virtual port (Hope this term is correct) and prioritize O365 traffic from that specific customer to MS.

Now,
Either the ISP are clueless, and do not know how to do it, or this is a very new thing for them and they really do not know how to do it.

The solution they want to sell us is a direct 100Mb connection to MS. Which is great but too expensive for us + an overkill of what the client needs. (MS and ISP are in the same PoP)

Anyone have an explanation about how to create a virtual port? Ideally, I would like to provide the ISP a step by step guide on how to create this port so it can be used by my customer.
If not a step by step guide, just explain them how to do it in simple words.

Any suggestions?

Thanks!