If you accidentally you don't send enough Crypto you have several options:
1. Send more funds to complete your purchase. If it has been requeued, it will be completed when you enter the front of the queue again.
2. Leave it be, and if you are lucky, maybe next time you enter the front of the queue the new conversion rate will make up for the difference and your purchase will be completed.
3. Leave it be, and if the conversion rate is not in your favor your purchase will never be completed, so at the end of the TGE your funds will be returned, minus network fees.