Economic efficiency gains are one potential advantage that Air Navigation Service Provider (ANSP) mergers might bring to the European Air Traffic Management system. An ANSP merger might also affect the use of airspace and consequently the level of congestion. In this paper, we analyze possible effects of an ANSP merger on traffic distribution i.e., route choices of an airline, congestion levels in different parts of the airspace, and consequences for airline's costs. We analyze four scenarios for an OD-pair market with a monopoly airline and two neighbouring ANSPs. For each scenario, we consider both cases: two ANSPs operate as competing providers and set different unit rates for the airline, and a merged ANSP operates as a monopoly service provider that sets a uniform unit rate for the airline. In all scenarios, we assume that the ANSPs are regulated based on a full cost recovery scheme. The numerical examples show that the different unit rates create an incentive for the airline to take detours in order to avoid the airspace with the higher en-route charge, resulting in additional fuel burn and emissions. Meanwhile, the uniform unit rate incentivizes the airline to choose the shorter route more often, leading to higher congestion levels on the shorter route and higher total congestion cost for the airline. However, in our numerical example, the decrease in the total distance-related cost outweighs the increase in the total congestion cost, leading to higher airline profits. We conclude that an ANSP merger that leads to a uniform unit rate can bring positive effects to airlines even if its potential efficiency gains are not taken into account.