How to model merit order (stepwise function)

I want to model an inverse merit-order (from highest price to lowest price) of products with varying quantity x(t) and price y(t) at time t. Products are included in the merit order until cumsum(x(t) = z(t)). Z is exogenously given. Note: if z(t) increases over time, the merit order should shift to the right to include the next available product with marginally higher price y(t).

Currently I have a vector MC(t) where 80 products are sorted based on their price, and a corresponding vector Q(t) with the corresponding quantities x(t). How to proceed? Thanks in advance!