Just discovered that 2018 has added the ability to add a sales tax total to the bottom of the report. Go to the "Layout" at the top ribbon of your SoftList report. Scroll down to the last line of the report (mine is labeled "TOTAL COST"). Right click over the words "TOTAL COST" and choose "Edit total". You'll be presented with a choice to add additional rows. Choose the sales tax row and click OK. SoftList will add a new line above the TOTAL COST that represents the sales tax.
BTW to set up the sales tax, click on the "Options" on the top ribbon of your SoftList report. There you can edit the name and percent of sales tax.