Well, I only had fees using DHL when the slow option wasn't available. It's actually DHL who charge the customs/tax and their own fee.
Looking just now, the only "slow option" offered is with Thailand Post, which I think is a different service and it goes up as the package gets bigger (which is understandable as before you had a small flat rate, which was probably too low; and the service was with Deutsche Post). So I don't know where that services stand regarding fees and customs. And looking at the stuff I ordered last for comparison, I think it would have cost me pretty much the same with the Thailand Post option with Tayda quoting me about 40 USD for shipping with this. So I think I'd rather pay DHL and the fees and have it quickly. Unfortunately he cheap slow option seems to be a thing of the past.