Checking their math, not just their checkout total
Sellers in many places are legally required to disclose mandatory fees before you enter payment details — the US FTC's rules against "drip pricing," the UK's Digital Markets, Competition and Consumers Act 2024, and the EU's Omnibus Directive all target exactly this. If the total this calculator gives you is noticeably higher than what actually showed up at checkout, something wasn't disclosed the way it should have been.
Run a checkout you're suspicious of through Kibbo's Hidden Fees Checker checklist to spot the pattern before you pay, not after.