You can also check if this is caused by a browser issue in case you can't still see a Save button. Start by using an incognito/private browsing session just in case the same issue appears. Use these keyboard shortcuts to initiate an incognito browsing session:
Ctrl + Shift + N for Google Chrome
Ctrl + Shift + P for Firefox
Control + Option +P if you're using Safari
Ctrl + Shift + P for Microsoft Edge
You can also delete cache or use any of browsers that I've enumerated above. They all work well with QBO.
Once done, go back to QBO and open the Estimates window to check if you can already see the Save button in the lower-right hand corner.
You'll also want to run an Estimate by Customer report after recording the transaction. This is for you to see the list of Estimates made for a specific customer and review them as well. Just go to the Reports menu and type in the name of the report in the Search bar.
Keep me posted if you still need more help with this.