I pay into my shares ISA over in internet and my cash ISA by post and both my providers tell me how much unused limit I have,so I don't think I would be able to put too much in.
Although it could be different with other providers.
you can only pay �3,000 into any single ISA account it doesnt phsically let you credit more.
If you subscribed to more then one ISA in any tax year and paid money into it i am afraid before to long you will hit with a bill from the Inland Revenue!