Function validateInvoice

Returns {
    customer?: {
        birthday?: string;
        city?: string;
        company?: string;
        country?: string;
        email?: string;
        firstName?: string;
        houseNumber?: string;
        id: number;
        lastName?: string;
        notes?: string[];
        phone?: string;
        postCode?: string;
        salutation?: string;
        street?: string;
        timestamp?: string;
        uid_number?: string;
    };
    id: number;
    isCanceled: boolean;
    items: {
        discount: number;
        id: number;
        name: string;
        price: number;
        quantity: number;
        taxRate: number;
    }[];
    paymentType: string;
    system_id?: unknown;
    timestamp: string;
    total: number;
}