I just got a macbook air . I wanted to purchase excel. it put $10.14 in my account. How often is this charged?

Read Subscriptions and Billing - Official Apple Support.

Microsoft charges $119.99 a year for their "Microsoft 365 Personal" subscription, so $10 a month seems about right. Add another 1.5% for sales tax? Who knows.

Microsoft's pricing model confuses me which is one of many reasons I don't use their products. Contact Microsoft for a definitive answer: Contact a third-party vendor - Apple Support

• Microsoft products: Visit the Microsoft website.

I suspect that is the price for a one-time purchase of Office Home & Student but it isn't at all clear in the pricing when buying through Apple (i.e., the identical description in the Top In-App list at two very different prices) -- and it could be for something else for all I know.

Only thing I know for certain is that I have a Microsoft 365 Personal subscription through Microsoft and pay $69.99 per year.

Yes, that would be a monthly fee for Microsoft 365

The following has other options directly from Microsoft: https://www.microsoft.com/en-us/microsoft-365/buy/microsoft-365

They also charge $119.99

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re: "Microsoft charges $119.99 a year for their "Microsoft 365 Personal" subscription..."

In the U.S. it is $69.99 per year. See: https://www.microsoft.com/en-us/microsoft-365/buy/microsoft-365

There are free and low cost equivalents to Excel. OpenOffice, LibreOffice, NeoOffice...

I use Numbers, which is free.