Why the jump in my cost for single Lightsail instance?

Thank you. Yes, when I started a new instance I inadvertently was upsold. Still Linux, but a virtual server with better CPU/memory than I need.

There is also a charge for static IP if they remain unused. Did you reuse the same static IP (which would make it free) or accidentally leave the static IP around but unused (which would be charged)?

This helped. I’m no longer being charged for the 1GB Linux instance. Still, after changing back to a 512 MB Linux instance, I'm far from my usual $3.50 monthly bill. 'AWS-Andy' commented that I should check to make sure I don't have any unused static IP addresses. I found one and deleted it, keeping only my current static IP address. Even so, my estimated bill for this month will be around $7.14, roughly double my usual bill. To be clear: I only have one $3.50/month instance running. All other instances have been deleted. And I have a single static IP address. No overages. What could I be missing? Thank you.

The forecasted amounts, as described in this document, are estimates based on historical usage. Therefore, there may be a discrepancy from the amount actually accrued this month. In other words, if you are removing unnecessary resources, the cost will be lower than the projected amount. https://docs.aws.amazon.com/cost-management/latest/userguide/ce-forecast.html

A forecast is a prediction of how much you will use AWS services over the forecast time period that you selected. This forecast is based on your past usage. You can use a forecast to estimate your AWS bill and set alarms and budgets for based on predictions. Because forecasts are predictions, the forecasted billing amounts are estimated and might differ from your actual charges for each statement period.

If the fees do not decrease after removing unnecessary resources, we recommend that you contact AWS Support by opening a case with "Account and billing". https://docs.aws.amazon.com/awssupport/latest/user/case-management.html

Thanks Riku_Kobayashi. Will watch this carefully and respond after next billing cycle.