Trying to downgrade subscription, keeps giving error message 81511, when I check the billing details, even though I don't change anything it keeps charging me, help.

You'll have to ensure that your billing information is accurate, Juan.

Error 81511 in QuickBooks Online (QBO) occurs when there’s an issue with the payment information entered, particularly the billing address, credit or debit card number, or expiration date.

To resolve this, you'll have to review your payment information, ensuring your billing address matches the one listed on your card statement and QuickBooks Online (QBO) account. Next, you'll want to remove any special characters, such as hyphens, commas, or periods.

You can refer to the article linked for step-by-step instructions: Fix “my subscription payment failed” issues in QBO.

Given that your subscription is being charged continuously, I recommend contacting our live support team directly. They can securely access your account and use their specialized tools to investigate the root cause of the error.

I've also included this article on managing your account, billing information, and subscription details effectively: Billing Hub for Small Businesses.

Feel free to post this post if you need further assistance.