gmailr | Access the Gmail RESTful API from R | Email library

I need some shiny wisdom, please. Below code works well in shiny server part; it sends an e-mail when user clicks actionButton("mailButton", "e-mail!") in ui part with a PDF report attached via gmailr. The only problem I have is that there is no feedback to the user whether e-mail has been sent. What I would ideally like to have is a message box mid-screen (maybe with overlay). When e-mail is sent it would tell so to the user who pressed the input$mailButton (and maybe the box would disappear with timeout of 15 seconds). This event approximately should correspond to the point of print("message sent") in the below code (this does get printed to the terminal). What would be a good way to do display user a confirmation that e-mail has indeed been sent and block inputs while sending is going on (which usually takes 4 seconds)?

I have been trying to send 2 images(have to add these in the mail body) and 2 attachments(2 excel files) in one email using the GmailR package but I've not been able to figure it out so far. I've already looked at the syntax from the documentation and this vignette - https://cran.r-project.org/web/packages/gmailr/vignettes/sending_messages.html

Based on the vignette above I've tried a combination of things mentioned in the vignette(attachment and image sections)to add two images in the mail body + two separate xlsx files. It works perfectly fine when I try to send one image/one attachment (as mentioned in the vignetter) but fails to take more than one image/attachment.