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I need the feature to add text to the price (inkl. 19% Umsatzsteuer)
If it is not possible to install existing extensions, is it possible to write my own extensions for magento-go?
Can someone guide me to some resources with the description for this task?
To answer your question, you cannot install regular Magento Extensions in Magento Go. However, for your specific question about the text, you can add it using a CMS Static Blocks. This article should get you started:
Note that with the upcoming Magento Go Platform, you will be able to easily include Go Apps into your store. Magento Go Platform will be available as part of the Magento Go later this year.