USAePay supports DMARC compliance via SPF.
Add 'include:spf.usaepay.com' to your SPF record. WARNING: as of 2021-11-10 use of this include will add 7 DNS lookups to resolve your SPF record, so you should only add this entry on a subdomain that is exclusive for use by this vendor. Use of this include on your primary domain will likely result in errors and widespread SPF authorization failure for your traffic in general. Contact USAePay support for more information.