So here's a hard-found tip about how to call a Smarty plugin function -- you delineate parameters with spaces, not a comma or a bar. You need to do this

{customer_functions expires=$cust[c].expires paymenttypeid=$cust[c].paymenttypeid assign='rowclass'}

This lets you visually show whether a row is overdue and what payment type they are using. Comma-separated would be a lot easier to read.

Let's see what else it takes longer to figure out with Smarty than it should.