body {
    font-family: Arial, sans-serif;
    background-color: #f4f4f4;
    margin: 0;
    padding: 0;
}

.container {
    width: 80%;
    max-width: 800px;
    margin: 20px auto;
    padding: 20px;
    background: #fff;
    border-radius: 8px;
    box-shadow: 0 0 10px rgba(0, 0, 0, 0.1);
}

header {
    text-align: center;
    margin-bottom: 20px;
}

header h1 {
    margin: 0;
    font-size: 24px;
}

.billing-form,
.billing-summary {
    margin-bottom: 20px;
}

.billing-form h2,
.billing-summary h2 {
    font-size: 20px;
    margin-bottom: 10px;
}

label {
    display: block;
    margin-bottom: 5px;
    font-weight: bold;
}

input[type="text"],
input[type="number"] {
    width: 100%;
    padding: 8px;
    margin-bottom: 10px;
    border: 1px solid #ddd;
    border-radius: 4px;
}

button {
    padding: 10px 20px;
    background-color: