Difference-in-Differences for a Regional Fee Change

A delivery service introduced a small-order fee in Region A in March. Region B kept the old policy. Monthly orders per active user:

Month     Jan    Feb    Mar    Apr    May
Region A  4.0    4.1    3.7    3.6    3.6
Region B  3.5    3.6    3.7    3.8    3.8

1. Using Feb as the pre-period and Apr as the post-period, compute the DiD estimate of the fee's effect.
2. Look at the full series. Is the key DiD assumption plausible here? Explain what you see and how it changes your conclusion.
3. Suggest a design that would be more credible given this data, and a placebo test you would run.