Envato has no taxpayer information about you (you don’t submited the W-8 form)
Without this taxpayer infos, Envato must treat you as an US-Author without any tax infos = 28% US Backup Tax on all your sales no matter of the nationality of the buyer.
I see you are from Portugal and Portugal has a double tax treaty with the USA of 10%.
- fill out W-8 form without foreign tax ID = 30% tax on US sales only
- fill out W-8 form with your Portuguese Tax ID as foreign tax ID = 10% tax on US sales only