In order to avoid having your formatted Product Descriptions overwritten in Shopify, be sure that Update Description is Off in Accumula Settings. In accounts on Startup (formerly Pro) Plan and above Update Description is set to Off by Default.
You can see what field is mapped to the Shopify description from Point of Sale in this documentation:
When a Product is created in Shopify the description will use the data from the POS field it is mapped to. With Update Description set to Off, you can edit the Description in Shopify without worry.
Many Point of Sale solutions, including Lightspeed, will remove all or most formatting in the long description sent by their API. To retain Shopify description formatting be sure to leave Update Description Off and manually edit descriptions on Shopify.