Is Uku always "should pay," and never just "pays?
Uku is "pays." E uku is "should/will/is expected to pay."
Is EXPECTED to pay?! Wow! That's pretty presumptuous!
Mahalo nui for that perspective!
The pros (Pukui and Elbert) make "e" a marker of imperative or exhortative mood. My amateur take is that it means "Here's what better happen:" E uku 'oe, You better pay/Pay! E uku kāua, We ought to pay/Let's pay. E uku 'o ia, She should pay.