I'm no expert, not even a 'Latvian speaker', but I think the <s> would be silent, that is absorbed into the <č>. The letter marks the word as nominative singular, like other nouns ending in <-s>, but is not separately sounded. Going through my little book, the only word I can find ending in <čs> is mačs "match", but that has no pronunciation in Wiktionary. However, Wiktionary shows that the <s> is absorbed in words with similar endings: īpašs "special" is [īːpaʃ] and plašs "wide" is [plaʃ]. So I presume mačs would be [matʃ]. I won't use IPA for Ermičs because there are two possible values for <E>.