Time series forecasting issue : negative value peak

Heeellooo there. Hope you're doing well.

Here's my problem: I was asked to do revenue forecasting for all of our shops. Was great, 7% of MAPE over 3months. I use a "fake" sarimax with Fourier terms to predict seasonality (we have yearly seasonality)

Then, upper management asked to do it at the each shop level. Here comes the trap : from time to time, shops must refund buyers to the extent that for that day, the data is negative. Sometimes, …

datascience fake forecasting issue management negative peak revenue seasonality series terms time series time series forecasting value