I would suggest filtering at the customer interface and then tagging the route with a community that indicates "I have accepted this route as a customer route and will transport it to and through the edge of my network". Here is some example code. Let me know if you need more help with this.
ON the ISPEdge router (facing the customer)
router bgp nnnn
neighbor x.x.x.x remote-as y
neighbor x.x.x.x Description customerxyz
neighbor x.x.x.x route-map customerxyz-in in
neighbor n.n.n.n remote-as nnnn
neighbor n.n.n.n Description IBGP connection to ISPCore
neighbor n.n.n.n route-map ibgp-out out
neighbor n.n.n.n send-community
ip prefix-list customerxyz-in permit y.y.y.y/20 le 24
ip prefix-list customerxyz-in permit z.z.z.z/22 le 24
route-map customerxyz-in permit 10
match ip address prefix-list customerxyz-in
set community nnnn:999
(Implicit deny any for Items not matching the route-map)
ip community-list 1 permit nnnn:999
route-map ibgp-out permit 10
match community 1
route-map ibgp-out permit 20
Statements To match and transport your non-customer ibgp routes go here
Ejay Hire
Network Engineer