You do have a sliding scale, in the form of the hosting cost pass through. Your average restaurateur is entirely unqualified to estimate the AWS hosting costs of your software in relation to his/her restaurant, so these costs are completely opaque. (As patio11 puts it, "Metered pricing is none of predictable, transparent, or fair.") You, on the other hand, probably have a very good idea of the costs relative to e.g. size or popularity of establishment. In the interests of transparent pricing, you ought to consider factoring these costs into your price and charge your customer a fixed predictable amount, tiered according to the relevant parameter.
