If you use single language site, then just rename the caption of a field in product field editing page or in order field editing page in backend.
If you use multilanguage site, then you can do the following:
1. Rename the caption of a field so: "Tax total" to "Tax_total".
2. Add to your language files /languages/xx-XX/xx-XX.com_ipricecalc.ini and /administrator/languages/xx-XX/xx-XX.com_ipricecalc.ini the variable TAX_TOTAL with translation:
TAX_TOTAL="translation of tax total"
Note, that uppercase symbols of variable (TAX_TOTAL) is usefull in language file.
Do so for each caption of the product field and for each caption of the order field.