I paid the difference in tax when I filed my 1040x. Now I received a bill for that amount PLUS the interest. Do I cancel the check I sent, then pay the full amount?

No, don't cancel (stop payment) your check. This is a timing issue. The IRS sent the letter after you sent your check but before your check was posted to your IRS account. If you agree with the additional interest amount that was charged, you may pay it. To pay online, click on Payments | Internal Revenue Service

If you want to talk to someone at the IRS to see if your check was posted, contact them. The IRS provides their contact phone number on the top right-hand corner of the notice or letter. You can also write to the IRS at the address in the notice or letter. If you write, allow at least 30 days for our response.

The location of the notice or letter number: You can find the notice (CP) or letter (LTR) number on either the top or the bottom right-hand corner of your correspondence. Reference: Understanding Your IRS Notice or Letter | Internal Revenue Service

[Edited 3-10-2020|8:37 am PST]