From a text I'm translating:
The current export freight rate that covers a range of shipments, including mineral fertilizer shipments, includes a 13.4 % surcharge.
Russian Railways officially charge the same rate as they charged back in the beginning of 2014. However, in late 2014 they increased this rate "temporarily" by 13.4% for a range of goods, to mitigate the losses caused by the fall of the ruble.
In Russian the word for this increase is "надбавка" ("an add-on", probably, when rendered literally). The Multitran dictionary offers a bevy of possible translations: надбавка.
Is surcharge a good option here? If not, what could be a better one?