If you have 'spare' connects (usually I don't use all of mine) then it might be worth sending a proposal if you think you can do the job using your own methods. For instance, people sometimes ask for a DXF file to be drawn using AutoCAD. You can produce the exact-same result using Solidworks, just the process of getting there will be different. For the portfolio -I basically have a selection of about 10 -20 previous projects, explaining the project, the process of creating it and the final outcomes. It doesn't have to be complex - maybe only a page, or three or four images for each project. Clients can then see your range of skills and experience and you can also highlight specific pages (or add extra pages) if you have something that's particularly relevant to their job. For the 5% fees - I'm afraid it's on a per-client basis, not your total earnings. Up to $500 per client you get charged 20%, then $501-10,000 you get charged 10%, then everything over that is 5%. So you need to do a lot of work for one person!
The 5% fees seems to be a long way from now. 10,000$ worth of work might be more than 2 or 3 months work for me. About the portfolio, so you make a short report pdf summary of your projects and you put them up on your portfolio to make it visible to your client. Is that right? I was wondering if there are any other sites which might be better for European market?
I personally made a PDF that was maybe eight pages - one intro, about five with one large project per page, then a few with multiple smaller projects per page. You can also make a portfolio website pretty easily on Wordpress or using a free portfolio site (you can also add portfolio pieces directly to your profile). It's a bit of a pain to put together but it's worth the time! I'm not sure on other sites but I'd be interested to hear too. I think Upwork dominates pretty strongly now.