Late payment advice

I am a very part-time freelance web content writer (3-6 clients a year), and I do all of my projects for clients who license the same software, so it's kind of a niche market. I work with the same website designer and we kind of have a pattern: I spend a month working with the client to write/map the site, and I finalize the content and send it to the web designer. He then takes over the rest of the project.

I always include a contract and payment terms upfront, and get written affirmation before moving forward. I require 50% up front, and 50% on completion of the project-- when I usually hand everything over to the designer. I specify that the package price includes one first draft, a conversation to go over major changes, and one final draft. I usually do an extra third draft, which is mostly just a small sweep up to make sure the client is happy with the final product.

With this latest client, they were unable to get me the items needed to fully complete the content (bios and employee information for the about us page, etc) by their desired timeline. Although it made me nervous to extend the timeline of my involvement in the project (I could easily imagine a lot of scope-creep), I agreed to do so. I got written confirmation that the final 50% would be paid when I released the content to the designer, and then I would complete the very specifically outlined sections when the client had the info.

I sent my first invoice along with a payment due date (2/17). 2/17 came and went with no payment or client correspondence. I emailed the client an invoice reminder email along with a copy of the invoice. I haven't yet received a response, and I plan on sending a third reminder email in one week. After that, how should I proceed? I have a great relationship with the designer, so I could call him up and let him know what's happening. I certainly won't authorize the client to use my writing if I'm not paid for it; the content license doesn't become theirs until it's been paid for. I could also send a late payment notice or a notice that the outstanding balance will go to collections. The client lives overseas, so it might be hard to collect, and since I work primarily within this niche I don't want to sour my relationship with this client. Is there a way I can get my money without resorting to threats or escalating the situation?

This is exactly why I was really reluctant to release the content without it being totally finalized-- I have a feeling the client is trying to withhold payment until all the sections are complete, which to be perfectly honest, could end up with him never getting me the additional info, the final parts of the project being abandoned, and him feeling like he doesn't have to finish paying me.