Are you frantically uploading and downloading files directly or indirectly via a third party platform (who are possibly charging you for the privilege) ? There is another way.
If you are supporting Drop Ship Vendor (DSV) then why not connect to the marketplace.
When selling via a marketplace you are placing your products on a third party platform and therefore you should be validating the data processed as you would with DSV.
So why not extend the reach of your EDI development. Can you see how this works back to thinking through your strategy - please see https://www.southwestdataintegration.co.uk/post/how-to-get-the-most-out-of-an-edi-development
I appreciate that eCommerce teams will have this all in hand but you are now dealing with a third party system. Surely it would be wise to apply all of the validations rules you have in your wholesale and DSV trading model?
Also, from my experience, the file formats and connection will change with each marketplace. This is a behaviour you tend to see when selling electronically on a wholesale basis.
You will also need to track these orders as you might find you have to pay some form of commission to the marketplace website. Again this is a similar behaviour you see when working in a wholesale environment and certainly something you will not have to do if you have your own website.
Options available to you
There are a vast array of applications you can use to access a marketplace website. These applications have to make money so how do they earn it? A flat fee? Commission on the order value processed?
Have you considered connecting directly, albeit through an EDI Managed Service Provider? What we need to remember is that this is a connection and transactional data.
I hope this gets you thinking, at a time when overheads are rising; costs need to be reduced.