Update: I checked in with Figma's Customer Support team, and they've received your request. They're reviewing it now under case: 2086116, and they'll follow up by email at the address tied to your Figma account with next steps.
Since this comes down to seat counts on your invoice not matching what your team actually used, Support is best placed to review those charges against your account directly.
For future reference, here's an overview of where to find and manage your plan, invoices, and seats: Guide to billing at Figma.