I understand that you're talking about the "Accepting Custom Orders" Feature from gigs tab.
So a buyer can contact and request a custom order with custom budget, deadline and with criteria.
It's not that buyer placing the order directly with custom pricing. Buyer is just sending the request directly to you to send him an offer if you can meet the budget, deadline and criteria.
Check the attachment like how I get.
Once you get the request from buyer, not it depends on you if you would like to accept the proposal and create an offer with his custom requirements of budget, deadline and criteria.
Even if you don't want to work with his requirements, send a professional reply and discuss in details. Never leave that message without responding as it's related to your Inbox response rate and time.
You can edit your gig and write that you can be available with google meet or other communication platform once the order placed, also in order requirements part.
Fiverr will hold your gig for a review, and based on your work category, if fiverr think they can allow you, then they will approve your gig.
Once fiverr approve the gig, then you can easily connect with your buyer outside by sharing that specific communication details.