[offtopic] question regarding inventory

Hi, i am a Distributor and my main business is done in offline market, however i had started selling on amazon and i found that i am not able to keep track of my real inventory, for e.g lets say i have a product named X and i have 10 pcs of it IRL, i sold 2 in offline market and 1 on amazon so IRL i have 7 pcs but as per my sales software it says i have 8. Now one issue is that i use a custom made sales/business software which is mainly made for offline business and if i record sales of amazon then i will have to pay double tax (one via amazon and one according to software as it will generate invoice), i had an idea that if there was an option to manually deduct/increase stock in software than it could maybe solve my problem but idk if thats practical. If anyone else has faced similar question then please help me out.