I'm associated with Flagship Technologies, Inc., the maker or FormCalc SST....

> I tried getting in touch with FormCalc SST support, but I did not get any response back,

You tried how? Email contact? Phone? 

We answer all emails. However, the first time you email us our mail server may send an autoreply which requires you to verify that you are a human, not a bot, who sent the email. So you may need to check your email account's SPAM filter, in case our autoreply got stuck there (i.e., the problem could be on your end). You can send to our Support address, [email address removed] to avoid the SPAM trap altogether.

And if you tried by phone, did you leave a message? We filter out junk/SPAM phone calls by requiring that you leave a callback message in many cases.

> and to me, the website looks a bit sketchy.

...a view which apparently is not shared by the many people who purchase from us and/or contact us through our Web site each month.

As for other calculation add-ons for QuickBooks desktop editions, the only two I know of are FormCalc SST and CCRQInvoice. Both are good products which have been on the market for many years (10+ for both, I think), and with a lot of users of each. There's some overlap of functionality between them, but major differences in how they work.

FormCalc SST calculatons are set up in the program's internal spreadsheet, which it applies "on the fly"  (in near-real-time) as you enter data on QuickBooks sales forms (invoices, estimates, sales receipts, etc.), invoked for the current form by pressing a function key. You get to see the calculation results before saving/completing the form, so it excels when each form has a different set of Items or calculations to do, as is typical in many sales situations.

CCRQInvoice processes calculations for a batch of sales forms (you select the range of forms to process), which means the calculations are not visible while you are entering/editing on a form, but afterward. That makes it a good choice for doing calculations on a group of very similar forms--such as for billing for utilities in a small municipality or water district, or billing for rent in an apartment complex.

But both products are capable of things like calculating total shipping weights for invoices, etc.