code: public Offer create(Integer amount, String currency, Interval.Period interval, String name) {
          return this.create(amount, currency, interval, name, null);
      }
docs: Creates an offer via the API.
  
  @param amount   Amount in cents &rsaquo; 0.
  @param currency ISO 4217 formatted currency code.
  @param interval Defining how often the {@link Client} should be charged. Format: number DAY | WEEK | MONTH | YEAR
  @param name     Your name for this offer
  @return {@link Offer} object with id, which represents a PAYMILL offer.