Excellent suggestion, let me work it in. Actually, perhaps a 50% tax on wire transfers with the revenue going to border protection. Probably most of the money transferred was earned "under the table." Or, am I being too cynical?
A tax would be in order if proper identification were not furmished. A 50% tax sounds about right but there are problems with imposing a tax.
For the tax to be given over to border enforcement, the tax would have to be a federal tax. The Congress would have to authorize it and administer that tax. Since the tax would be received at primarily small retail outlets, the administration of the tax would not be straightforward, e.g. there is no current tax taken in on a daily basis that is federal in scope. State sales taxes could be imposed though with a federal surcharge for lack of proper ID.
Local 'Checks Cashed' and 'Dinero a Mexico' wire services could circumvent the tax by having a front person with proper ID sned the cash. That would then be a tax enforcement problem.
It may be easier to administer without tax by requiring retail outlets only to keep photostats of IDs with copies of wire transfers. A random audit of records could then assess whether the business outlet was in compliance. Fines and penalties or threat thereof should keep retail shops inline.
Of course there can be no perfect system but the idea is to make it increasingly harder for illegals to profit from jumping the border and also funding the next batch of border violators.