Like it has been said above:
1. Email was never meant for large file attachments. FTP is. FTP is not that complicated, and there are free simple FTP programs out there to direct your customers to.
2. Dont blame your ISP. It is possible they wont up your limits as a matter of policy, or it is possible that their mail server software is not capable of per-mailbox limits.
3. As an alternative to FTP, there are web based file sharing services you can pay for. These use the web browser to easily upload/download files.
4. If it just HAS to be done in email, you have three options.
a) You can pay for a simple email account from some email provider, with prediscussed limits, and use that email account for your customers to send the large attachements. This way, you only incur the expense of 1 email account outside of your ISP.
b) You can find another ISP strictly for email services, or find another ISP for all your services and agree on mail size limits in advance.
c) You can set up your own mail server on your premises. It would be good to have someone tech savvy if you are going to depend on it. I too, would recommend Exchange 5.5/Small Business Server. It is rather inexpensive for a small office to afford, and works VERY WELL, no matter what the nay sayers wish to say. Keep in mind, you will have to deal with securing your mail server from spam, adding antivirus solutions, etc..etc.. which become complicated, and lead you to go with an ISP in the first place.
Please keep in mind, that a 3MB limit is not all that uncommon, and several of your clients will also have these sort of limits as well, leading one again, to FTP.... your easiest and cheapest solution.
Might I recommend, that you find a free FTP client, and then write up a nice short Word document, that explains the location of this client for download, how to install it, and how to upload files to your FTP server. This way, written in a plain and simple format, you can offer this as a solution for your clients. You will need to cover this from the PC and MAC perspective, in a graphics world.