Bear with me, because I'm not a seller, but I think what you will need to do is edit each template you have saved, and go to the section that specifies postage. You'd select the Flat postage option rather than Calculated postage option, and then save the template. (It seems to me that the Flat option is the default if you haven't already specified Calculated, if you create a new listing. I've just checked that, and it's certainly the case with me.)
(If anything I've said is complete and utter tosh, I know an experienced seller will come along and correct this!)
Here's a screenshot: