forecast monthly shipment (time series) for 300 products individually in R
It is difficult to answer this question with the limited information provided here. Nonetheless, I have following recommendations.

Do you have a (data backed) reason to believe that you do not need 300 different models? For example, are the shipments of different products correlated? Alternatively, can you instead model the problem at higher levels like product categories? If either case, you can take a look at hierarchical time series modeling. This journal article by Rob Hyndman, author of forecast package in R can get you started on hierarchical time series modeling.