Contingent valuation studies of people’s willingness to pay (WTP) for ecosystem services are frequently used to inform the social benefit-cost analysis of environmental protection measures. Though contingent valuation is generally accepted in this context, response anomalies exist. Drawing on the anchoring-and-adjustment heuristic in psychology and context effects literature, we examine the impact of subtle design variation on people’s WTP. In a split-sample national survey of WTP to prevent coastal environmental damages from oil spills, different payment card elicitation formats significantly affect mean WTP, at most by 43%. This underscores the importance of a research agenda on the effects of subtle design variation on environmental value estimates.