A frequently discussed idea in the context of carbon pricing is to return the revenues from these taxes to households as lump-sum transfers (“climate bonus”). Neoclassical theory predicts such transfers are fungible and do not weaken marginal incentives, yet behavioral theories suggest people may net rebates against fuel costs and treat them as earmarked. We test whether households understand the marginal incentives of a CO2 price combined with a lump-sum rebate. Study 1 is an incentivized quiz with a representative sample from the German state of Lower Saxony (n=4,098) that asks participants to compute trip-specific cost changes under a CO2 tax with and without a monthly bonus. While ~90% answer correctly under the tax alone, correctness falls to 13–32% when a bonus is mentioned, and most respondents choose a “netting” response. Study 2 is a personalized survey experiment with car owners (n=1,337) that fixes the marginal price increase and varies rebate size and label. We find substantial earmarking and partial reversal of prior fuel cutbacks, consistent with mental accounting. Overall, the results suggest that households do not fully understand marginal incentives. A policy mix designed to maximize political support for higher CO2 prices may therefore have the unintended consequence of substantially weakening the perceived incentives to reduce fuel consumption.