Direct answer
Mercury fits a US-registered company that wants banking and finance workflows in one digital console.
The evidence-backed core is checking and savings through a customer-specific partner bank, with ACH, domestic and international wires, mailed checks, bill pay, invoicing, debit cards, team permissions, approvals and accounting connections. The core tier is listed at zero dollars per month with no Mercury monthly, overdraft or minimum-balance fee. That does not make every action free: foreign-currency transfers and card purchases, premium processing, subscription workflows, API mass payments, Treasury, IO conditions, operators and third parties can introduce separate costs.
The strongest fit is a company whose money already moves electronically. Mercury does not accept cash deposits through its partner-bank branches. Physical debit cards can withdraw cash at compatible ATMs, but the current published per-card and business-wide ceilings, business-day reset, operator charges, network rules and currency conversion still apply. A retail business that regularly takes notes and coins needs a documented cash-deposit path elsewhere rather than assuming a partner bank's branch will accept a Mercury deposit.