We prove gauge-invariant uniqueness theorems with respect to maximal and normal coactions for C*-algebras associated to product systems of C*-correspondences. Our techniques of proof are developed in the abstract context of Fell bundles. We employ inner coactions to prove an essential-inner uniqueness theorem for Fell bundles. As application, we characterize injectivity of homomorphisms on Nica’s Toeplitz algebra T(G, P) of a quasi-lattice ordered group (G, P) in the presence of a finite nontrivial set of lower bounds for all nontrivial elements in P.