You most likely have your pricing set to Separate Product Pricing in your Inventory Settings and/or you are using Customer Groups.
- Go to Settings
→ Inventory Settings
- Scroll down until you see Product Option Prices. Make sure you have it set to Added to base product price.
- If you are using Customer Groups, make sure you have the group set to Exact Amount and No Discount.
- Go to Customers → Customer Groups
- Make sure you have Product Price set to Exact Amount and the Option set to No Discount.