Basically, you cannot use both the records at the same time. Either you would add the A record or the CNAME record, but not both.
So if your customers wants to map their domain to the sub-domain, they will either add the A record which requires the use of the IP address or the CNAME record which is normally the host record of your server.
Also, if your customers add a CNAME record, then they will not be able to use the email services. This is the major drawback of using the CNAME record. So its always advisable to add a A record.
Hope I answered well.