收到canada customs and revenue agency的reminder notic：

You have (an) overdue return(s) as indicated below. You have to file the overdue return(s) at once, using the return form(s) we sent to you.

You have to file a return for each filing period. If you do not have taxable sales, tax credits, or rebates that apply to a filing period, complete and file a return by entering zeros on the appropriate lines.
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即刻打电话给canada customs and revenue agency,被告知需要添个表，退gst给政府。

问题1: 印象中如果全年business亏损（连代收的gst也亏进去了），我收的gst就应该不用退了。正确否？

问题2：taxable sales, tax credits, or rebates 各指什么？

问题3：是否有大侠收到类似信件，如何处理？谢谢！