1) No, you can enter the full amount of Startup Costs even though your income was only $3900.
2) Don't enter it under supplies, enter it under Startup Costs. That statement means that the 'default' rule is to amortize it over 15 years. But you have the option to make the election to use up to $5000 in the first year you are open. When you enter the full amount in the Startup Cost section of the program, the program will ask you a few questions and automatically apply the $5000 and then amortize the rest.
