Odd plugin incompatibility

I have installed nextgen gallery and set up a slideshow. This works beautifully.

Then I tried to install “web Invoice” and I get a memory exhausted error. I tried uploading it as a zip file and I get a message that the file size is too large for upload (which it clearly isn’t).

After trying all the php.ini allowed memory tricks – still nothing worked. Then I started turning off other plugins. and I see something interesting.

If nextgen gallery is deactivated I can activate web invoice. Then when I try to reactivate nextgen gallery I get a message that it requires php 5.2 (which I have) and won’t activate.

Deactivate web invoice and then I can once again activate nextgen but the invoicing plugin now sends the memory allowance fatal error again.

Where should I look to get these two to work together?

Alternately – is there an invoiceing plugin that works with payflo or paypal advanced that I can try instead of this?

thank you for your time.