Despite their potential to advance financial inclusion and improve business outcomes, digital payment solutions have low adoption rates among retail merchants in developing economies. Prior interventions have tackled pre-purchase frictions such as awareness and affordability, showing limited success. We instead examine post-purchase frictions in the merchant onboarding journey, focusing on installation complexity and value uncertainty. We implement a randomized field experiment with 479 cash-only retailers in Mexico, testing interventions modeled on Customer Success Management (CSM)—a marketing function widely used by B2B technology companies to onboard and retain clients, but not previously examined causally. In the first treatment group (T1: N=159), we tackle installation complexity by introducing a CSM to support System Integration for a provided digital payment solution. In the second treatment group (T2: N=162), we further tackle value uncertainty by adding a second CSM to support Value Realization. Relative to the control group (N=158), long-term adoption increased by 21.4 percentage points in T1 and by an additional 13.4 points in T2. Mechanism evidence shows that System Integration primarily operates on the extensive margin of ensuring a functioning device in-store and substitutes for missing digital capacity, which constitutes merchants’ digital savviness and the local installed base of cardholders. By contrast, Value Realization operates on the intensive margin of usage and complements existing digital capacity. Cost-benefit analyses and spillover analyses support the scalability of our interventions. These findings offer insights into promoting the diffusion of digital payments.