Easy Digital Download has a shortcode for the purchase button/link, called purchase_link.

By default, FastSpring will change the price in these shortcodes to correct prices (convert the currency and apply optional taxes for each visitor). If you want to disable that functionality, then you need to use the class attribute in the purchase_link shortcode. By adding the js-pp-edd-fs-show-original-price class to this attribute, the original price defined in EDD for this product will be shown.

[purchase_link id="94" text="Add to Cart" style="button" class="edd-submit js-pp-edd-fs-show-original-price"]

* The edd-submit class is there by default, so you have to include it as well.