Okay, I've got a comprehensive answer for you, that should cover all bases. I'm assuming you're a guy, by the way.
Firstly, for underwear, I wouldn't bother with Merino. Too expensive on the cost-benefit scale, plus it could be rather itchy. The best camino underwear I've found is Uniqlo AIRism - a synthetic. It's dries super quick, really easy to wash, and incredibly light. Really, just the perfect hiking / travel underwear. Make sure you go for the AIRism, which is a special synthetic, and not the plain cotton, which would be terrible to hike in.
Discover the new selection of AIRism Boxer Shorts at UNIQLO online. Select from a variety of styles and colours to suit your style.
www.uniqlo.com
You can find Uniqlo in the US too.
However, as far as shirts / tops go, Merino should be your choice for day-to-day wear. Merino needs to be washed once every few days; synthetics, daily. Consider this has an impact on your camino washing costs, plus free time. Then add in all the known benefits of cooling, warmth, wicking, etc. You need merino next to your upper body.
Yes the prime brands are expensive BUT ... Decathlon! Yes, Decathlon do their own brand Merino wear and it's totally affordable and every bit as good as the top brands. Less than 20 Euros a piece. If you don't have a decathlon where you are, locate the first one in France / Spain and make that your first stop. There are actually Decathlons on the Camino. There's one in Irun and another in San Sebastian, for instance.
£19.99 - MEN APPAREL OUTFIT TRAVEL TREK - M LS WOOL TS - TRAVEL100 - BLU - FORCLAZ
www.decathlon.co.uk
For sleeping, if you're like me, you'll find merino too itchy, so I would bring a plain polyester or polycotton T-shirt for night times so you're not scratching yourself all night long!