Welcome to the site. I'm PDtax, and was prompted to assist by the site.
This is a new program started by IRS late last year for taxpayers who receive 1099-K from their credit card and bank transaction processors.
IRS uses four form letters for these inquiries, suggesting remedies for repairing the discrepancy.
You can review your records, including a review of the gross sales line on the business tax return you filed, and compare that amount to the 1099-K you received. That is likely the reason for the letter.
Once you review your records, you can verify the correctness of the return you filed. If it is correct, a reconciliation comparing the amount IRS expected with the amount reported will likely satisfy IRS.
If instead there may be additional receipts to report, I suggest a review with your tax professional who prepared the return, and review your options.
Thanks for asking at Just Answer. I will be glad to address any follow up you may have. If this covers things, positive feedback is appreciated. I'm PDtax.