<think>
Okay, I need to implement the tier_cost function. Let's see the problem statement again. The function should calculate the cost of units based on a tiered system. Up to the allowance, it's charged at the first rate. Anything beyond that is charged at the second rate. Also, if units is below zero, it should reject with an error.

First, I need to check if units is negative. If so, raise a ValueError. That's straightforward.

Next, determine how much of the units are within the allowance. If units is less than or equal to the allowance, then the cost is units multiplied by the first rate. But wait, what if the allowance is zero? Well, the problem says that if the allowance is below zero, it's treated as no allowance. Wait, the problem says that the allowance below zero is not stated. Oh, the user mentioned that the allowance below zero is not specified. But the function's parameters are allowance: int. So, perhaps the allowance can be zero or negative. But according to the problem, the usage inside the allowance is charged at the first rate. So, if allowance is negative, maybe it's treated as zero? Or maybe the allowance is supposed to be a non-negative value. But the problem says that the allowance is not specified. Hmm.

Wait, the problem says that the function should reject a number of units below zero. But what about the allowance? The problem says that if the allowance is below zero, it's not stated. So perhaps the allowance can be any integer, but the function should handle it. But the problem says that the usage inside the allowance is charged at the first rate. So if the allowance is negative, maybe it's treated as zero? Or maybe the allowance is supposed to be non-negative. But the problem doesn't specify. However, the function's parameters are given as allowance: int, so perhaps the allowance can be any integer, but the function should handle it. But the problem says that the allowance is part of the parameters, so maybe the allowance can be zero or negative. However, the problem says that the usage inside the allowance is charged at the first rate. So if the allowance is negative, then the allowed usage is zero? Or maybe the allowance is supposed to be non-negative. But the problem doesn't say. So perhaps the allowance is allowed to be negative, but in that case, the allowed usage is zero. So, for example, if allowance is -5, then the allowed usage is zero. So the first rate is applied to zero units, and the rest is charged at the later rate.

But how to handle that? Let's think. The allowance is the maximum number of units that can be charged at the first rate. So if the allowance is negative, then there's no allowance, so all units are charged at the later rate. Wait, but the problem says that the usage inside the allowance is charged at the first rate. So if the allowance is negative, then there's no allowance, so all units are charged at the later rate. Wait, but that's not correct. Because if the allowance is negative, then the allowed usage is zero. So, for example, if allowance is -5, then the allowed usage is zero. So, if units is 10, then the first rate is applied to 0 units, and the rest 10 units are charged at the later rate. So the total cost would be 10 * later_rate.

But how to compute that. So the formula is:

if units < 0: raise error.

else:

allowed = min(allowance, units) if allowance is positive? Or allowed is the minimum between allowance and units? Wait, no. The allowed is the allowance.