The single most common misreading of a vision plan is treating the contact lens benefit as coverage in the health insurance sense, where the plan pays a share and you pay a share of whatever the total turns out to be. Vision materials benefits do not usually work that way. The plan commits a fixed number of dollars per benefit period, that number is applied to the order, and everything above it is yours. If the order costs less than the allowance, the difference is generally not refunded and does not roll forward.
That structure has a consequence people rarely price out. Because the allowance is fixed rather than proportional, its value depends entirely on the price of the order it is applied to. The same allowance applied at a seller with higher list prices buys you less than the same allowance applied at a cheaper seller, and a large enough cash discount elsewhere can beat the benefit outright. The only way to know is to build both totals for the identical prescribed product and annual quantity, then compare the amounts that actually leave your account.
A smaller number of plans carry a covered-in-full contact lens feature, where an annual supply of elective lenses is covered without an allowance cap. That is a plan design an employer buys, not a standard entitlement, and it is worth checking for specifically rather than hoping. If your benefit summary does not describe one, assume the allowance model.