There's a quick-and-dirty way of doing things: if this is a largely standalone WordPress and price calculator + emailer script, note that you can put an arbitrary PHP file in the root directory (or actually anywhere) on your site's file/dir tree, and unless you have rewrite rules and/or a server config to the contrary (note: you likely don't) then it can simply be accessed independently.
For example, if you have a
test.php script and your WordPress site is available online at https://example.com, and you plop test.php in the root directory of your site (e.g. where you see files like
wp-config.php and folders like
wp-content/) then you and your site's visitors can access
test.php from a browser at: https://example.com/test.php. Other forms served up, either by WordPress or completely independently from WordPress, could POST to
For a beginner-friendly WordPress way of setting up contact-type functionality, you could use one of the many contact form plugins out there. A popular option is Contact Form 7: https://en-ca.wordpress.org/plugins/contact-form-7/. There are dozens of similar options.
A great commercial forms option that can be used to create dynamic forms with conditional fields, etc is Gravity Forms: https://www.gravityforms.com/
It sounds like your script might be doing two things: there's a "price calculator" and there's a thing that "emails your company". If you need to integrate a calculator with your theme, you could try implementing it directly into your theme's template files, or perhaps creating it as a Widget: https://developer.wordpress.org/themes/functionality/widgets/. If your theme defines "widget areas" your widget could be placed anywhere within them easily from the wp-admin.
Hope this helps! Cheers!