I'm using a bitcoin wallet for managing all my transactions. For example, If I have bitcoin 2, 3 and 5. Firstly, I buy a product with price 5. Bitcoin wallet will create a transaction from 2 and 3. When my first payment haven't verified yet, I make second payment by a product with same price is 5. This time will bitcoin wallet choose 2 and 3 or bitcoin 5 for my second payment ? I ask this because first payment haven't confirmed yet.
If you have enough Bitcoin in your balance it will be all good. Double-spend transactions should only really be made maliciously by reusing the same inputs. If you are using a known wallet you should not have any issues.