freq=365.25/7 means you are assuming a year-long seasonality in your data. Since your results don't look good, try removing the seasonality by setting freq=1.
You can also try using the ets and auto.arima functions with freq=1 to see what kind of results they give. If the seasonality of your data is not strong, using a non-seasonal model will likely give better results.
You don't have a ton of data, but you could perhaps hold out a test set of 1 year to compare the tbats, ets, and auto.arima predictions.
If you have an external data that might affect sales (e.g. holidays or promotions) you can include that data in the auto.arima model, but not tbats or ets.
Also, 243334 on 3/1/2015 vs 556637 on 3/8/2015 is a pretty huge jump for 1 week: are you sure you don't have a data quality issue?
