The Bank of Canada 2015 Retailer Survey on the Cost of Payment Methods faced low response rates and outliers in sample data for two of its retailer strata: chains and large independent businesses. This technical report investigates whether it is appropriate to combine these two strata to produce more accurate estimates of the total private cost to large businesses of the main payment methods. It uses two approaches to compute the total cost. First, a sample-based approach assumes consistency of some sample ratios and calibrates the sample to the known population totals of auxiliary variables. Second, a model-based approach uses outlier-robust estimation methods. The results show that, unlike for payments by cash and debit cards, there is relatively little difference between the approaches in determining the cost of payment by credit card. The model-based approach is recommended because it does not allocate the retailer’s total costs in advance between fixed and variable costs, and it uses outlier-robust estimation methods.