Hi there HDHorne,
Thanks for reaching out about your invoices. QuickBooks Online is a great tool able to assist in creating and sending forms for your customers like invoices and etimates. I can point you in the right direction for assistance with the mobile app crashing.
Thank you for trying the steps you listed. For further assistance with the mobile app crashing every time you try to turn an estimate into an invoice, I recommend reaching out to the QuickBooks Online support team outside of the Community. They'll be able to assist with further steps to resolve the error, and collect the necessary information to notify the engineers if the error still persists, all while keeping your personal information secure. You can reach them using these methods:
Schedule a Callback or start a Chat: click (?)Help in the upper right > type and enter "Contact support" into the QB Assistant > click Contact Us > explain your situation > click Let's Talk > choose Get a callback or Start a Chat.
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if you have any other questions, feel free to reach out here.