Multiple approvers in invoice approval spreadsheet

Summary:

We need to setup invoice approval using multiple approvers who approve the invoice sequentially. The approvers are not in a hierarchy. The approvers are also not static named persons, so we cannot use approver groups. In fact, what we try to do is mimic a project hierarchy. Dependent on the invoice amount, either only the Project manager has to approve or other people have with a role in the project have to approve as well. In order to represent these other approvers, we use the project standard cost collection user defined attributes in which we select the right approvers.