This study elicites consumers' willingness to pay for an environmentally sustainable good that might not typically be purchased on its own. We isolate the value of these perfectly-complementary, auxiliary goods by endowing consumers with the complete good and giving them the opportunity to pay to upgrade the auxiliary component of the good. We employ a Becker-Degroot-Marschack auction design in a market setting to determine consumers' actual willingness to pay for the auxiliary good. We find consumers are willing to pay a $0.67--$1.12 premium for a bioplastic plant container over a traditional plastic one.