Consumers set mental accounts and purchase products that provide greater benefits using payable amounts as a reference, but it is not easy to evaluate the gains relative to the costs paid for product purchases. This is because consumers experience decoupling in purchases, where the costs they have paid are not linked to the benefits. Decoupling frequently occurs in promotions for bundled products where it is difficult to calculate specific benefits relative to specific costs, yet research on this topic has been limited. The purposes of this study are as follows. First, we aim to explore the influence of bundle product type as a factor affecting decoupling. Second, if bundled products have a positive effect on decoupling, we aim to explore ways to utilize consumers' mental accounts as a means to activate decoupling. Specifically, we distinguish bundled products by price presentation method and examine through experiments what role the existence and ambiguity of consumers' mental accounts play as moderating variables in the effect of these on the evaluation of bundled sub-products. The research findings are as follows. First, the effects of bundle product type on decoupling were found to differ. Specifically, mixed bundles had a greater effect on mental accounting decoupling than pure bundles. Second, the effect of mixed bundle product type on the evaluation of bundled sub-products was found to be moderated by mental account characteristics (existence and ambiguity). Specifically, when a mental account for the sub-product exists, mixed-leader bundles had a more positive effect on sub-product evaluation than mixed-joint bundles, while when a mental account does not exist, mixed-joint bundles had a more positive effect than mixed-leader bundles. Furthermore, when the mental account for the sub-product is ambiguous, mixed-joint bundles had a more positive effect on sub-product evaluation than mixed-leader bundles, while when the mental account is not ambiguous, mixed-leader bundles had a more positive effect than mixed-joint bundles. Based on these research findings, theoretical and practical implications for marketing that utilizes consumer mental account characteristics in promotions using bundled products were presented.