Your robots.txt and sitemap.xml file are automatically generated by the Big Cartel platform. These files are correctly located at:
http://your-shop.bigcartel.com/robots.txt - or -
http://your-shop.bigcartel.com/sitemap.xml - or -
These files are optimised for your Big Cartel shop and are dynamic, i.e. they automatically update when you make changes to your shop products and content.
These files can not be edited or deleted and they do not need to be.
For more information relating to SEO, refer to What should I be doing about SEO?