I amended my return but still received a check for the amount before I amended it. Was my amended refund not processed?

That is correct- amended returns take longer ((amended returns take up to 16 weeks to process) and efiled returns are processed as they are received.  Your amended return should have given you the total with the refund from the first return included in the calculation- on the amended return, if you owed money you must send it back, but if you had a refund you will receive a check from the IRS after it is processed.

where do I mail the difference between the original and amended refund

Look at this page to find the correct address.  <a rel="nofollow" target="_blank" href="https://www.irs.gov/filing/where-to-file-addresses-for-taxpayers-and-tax-professionals-filing-form-1040x">https://www.irs.gov/filing/where-to-file-addresses-for-taxpayers-and-tax-professionals-filing-form-1040x</a>

My cousin filed a return but had to amend it. They approved her original amount and gave her a date for 4/18 WMR still says the old amount was approved but what about her amended return
The amended return is less
najar00 a few seconds ago

She will want to send the difference of the old and new refund. (She will receive the refund from the original return as planned)