A customer offset may be determined with respect to a customer who has previously traveled to one or more merchant locations. A merchant location offset may be determined with respect to multiple customers who have each traveled to a particular merchant location. Upon determining that the customer is in transit to the merchant location, a generic ETA to the merchant location may be determined. A customer-based ETA for the customer with respect to the merchant location may be determined based on the generic ETA and the customer offset. Moreover, a merchant location-based ETA for the customer with respect to the merchant location may be determined based on the generic ETA and the merchant location offset. Based on the refined and more accurate ETAs, the merchant location may be instructed to begin assembling items for pick-up by the customer.