You can use a domain that you purchased on Namecheap to point to your OfferingTree site by setting what is known as a CNAME record. Here are instructions from Namecheap to set up a CNAME record: https://www.namecheap.com/support/knowledgebase/article.aspx/9646/2237/how-to-create-a-cname-record-for-your-domain/
For the host, please enter www and customdomains.offeringtree.com for the value.
Ensure that you don't already have a CNAME record for www and there are no conflicts. Once you have set this up, please contact us at firstname.lastname@example.org and let us know the domain you would like to use, and we'll finish the setup on our end.
Additionally, we recommend adding an "URL Redirect Record" so users that type in your domain without a www prefix are redirected to the www version of your domain. For example, if your domain is example.com we please add an URL Redirect so example.com goes to www.example.com.
For the host, enter @, and for the value enter www.yourdomain.com (replacing yourdomain.com with your actual domain)