Hi @Susan E Aldridge, welcome to Figma Forum and thanks for flagging this.
I can't pull up account or billing details from this public thread, so I've connected you with our Customer Support team to look into the charge directly. Your case number is: 2116034 — they'll follow up at the email tied to your Figma account.
While that's in motion: On the Professional plan, a Full seat is $16/month billed annually and $20/month billed monthly, so a difference like this often comes down to the billing cycle on a plan. I can't confirm which applies to your account from here, but you can compare the two rates on the Figma pricing page using the monthly/annual toggle, and this guide to billing on the Professional plan walks through how the monthly and annual options work: Manage billing on the Professional plan.
If you can note where you saw the $16 figure (e.g. the annual rate on the pricing page, a specific plan, or somewhere else) that'll help Support line it up against your account when they reach out. 🙏🏻