Non-Americans (EU?), how are you handling taxes on Arena Directs?

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So I (Czech Republic) just got the tax form from WotC and idk what to do? Have sold all but one of my FDN boxes, all MH3 ones (BLB still waiting to arrive) at somewhat less than $250/trophy. Should I expect the US government to come after me? I heard on Arena Open cash they deduct the 30% automatically and you have to apply to get the difference between your country’s tax rate.
I’m not asking for you to lead me through the process, but anything like past experiences with overseas taxation on Magic winnings would help a lot. Thanks in advance!