Meaningful predictive analytics for small (n=114) dataset with just 1 explanatory variable and 1 response variable?
Create additional features from the given features. From the Monday "date" feature, the features ("month","year") can be found. Additionally, months of the year contain either 4 or 5 Mondays, so a "week" feature can be created with a value 1...5 that can be used to represent the week of the month that the Monday occurred on. Additional features can be created by calculating trends (Average Shipments for the previous N shipments,...etc.). This gives the algorithm more information to work with and should provide more refined results.