Can I use the time and the person-hours/facilities/services provided by my customers to cover the co-funding part?

Yes, two cases are possible:
1) If  the customer/user provides facilities, manpower, etc,  and they are not subcontractors, these costs shall be detailed in the Exhibit A of the Prime PSS forms as external costs.
If these costs exceed 20.000 Euro, the Customers/Users shall provide separate PSS form (PSS A1, A2, Exhibit A) detailing the costs they will incur and that will not be charged to the Prime.
Eligibility of such costs will be assessed by ESA, however as a general principle they can be eligible if required for the project, fully dedicated to the project and not entangled with normal business.
2) If the customer/user is a subcontractor, they can charge this cost in their PSS forms asking ESA zero co-funding. This cost will go in the overall bucket of the activity costs.